In his first months back in office, U.S. President Donald Trump has threatened to use military force to seize Greenland and the Panama Canal, suggested that the United States could take ownership of Gaza after the expulsion of two million Palestinians, and demanded that Ukraine give up territory to Russia in exchange for a cease-fire. These acts and statements might appear to be just a handful of examples of Trump’s typical wide-ranging and hyperbolic bluster. But in fact, they all form part of a cohesive assault on a long-standing principle of international law: that states are prohibited from threatening or using military force against other states to resolve disputes.
Before the twentieth century, legal theorists believed not only that countries could wage war to seize others’ land and resources but also that in some circumstances, they should. War was considered legal, the primary way to enforce national rights and resolve disputes between states. That all changed in 1928, when nearly every country in the world at the time joined the Kellogg-Briand Pact, agreeing that wars of aggression should be illegal and territorial conquest prohibited. The 1945 UN Charter reaffirmed and expanded that commitment, putting at its core a prohibition on the “threat or use of force against the territorial integrity or political independence of another state.” Having discovered that merely agreeing to prohibit war was not by itself enough, states then went to extraordinary lengths to design frameworks and institutions to cement this essential rule, leading to the establishment of a new legal order that elevated economic tools over military might to ensure peace.
As a result, wars between states became far less common. In the 65 years after the last settlements of World War II, the amount of territory conquered by foreign states each year plummeted to less than six percent of what it had been for just over a century before the world first outlawed war. The number of countries tripled from 1945 to today, as states no longer feared that they would be gobbled up by more powerful neighbors. And countries traded more freely with one another, knowing that the wealth they accumulated was less likely to be plundered by other states. The world became more peaceful and prosperous.
The influence of the prohibition on the use of force had already eroded somewhat before Trump returned to office. In 2003, the United States invaded Iraq, justifying the war by claiming that Iraq had weapons of mass destruction that it did not possess; China has spent the last decade building military bases in contested areas of the South China Sea; and Russia’s full-scale invasion of Ukraine in 2022 set off the largest land war in Europe since World War II. But Trump is shredding what is left of the norm against using force. Until now, the United States had played a critical, if imperfect, role in maintaining and defending the postwar legal order. The resilience of that order depended less on total compliance with international law than on a shared set of expectations about how other countries would behave: even if a country was not itself committed to the UN Charter’s prohibition on the use of force, it knew that violating the norm would likely trigger condemnation, sanctions, and perhaps even lawful intervention from the United States and its allies.
Now, that expectation is gone. Trump is not merely abandoning the United States’ traditional role in defending the prohibition on war and, with it, conquest. He seems to want something more: to restore war or the threat of it as the main way that states resolve their disagreements and seek economic gain. Other countries are already signaling an acceptance that the norms have changed. Israeli Prime Minister Benjamin Netanyahu appeared to endorse Trump’s musings about Gaza, and Panama chose to placate the American president by accepting deportation flights of non-Panamanians and signing an agreement allowing the United States to deploy military personnel along the Panama Canal. Amid Trump’s threats to permit Russian President Vladimir Putin to annex parts of Ukraine, Kyiv inked a deal with Washington giving the United States access to its rich mineral resources. If left unchecked, the erosion of the prohibition on the use of force will return geopolitics to a raw contest of military power. The consequences will be grave: a global arms race, renewed wars of conquest, shrinking trade, and the collapse of the cooperation needed to confront shared global threats.
ENTRENCHED WARFARE
For centuries before World War I, war was a legally recognized means by which states resolved disputes. The outbreak of war did not constitute a breakdown of the international order—it was the order. In the absence of a global court to adjudicate international conflicts, sovereign states had the authority to enforce their rights as they saw fit—namely, by going to war. States set out their legal reasoning for attacking other states in “war manifestos.” Any legal grievance could serve as a just cause for using military force: property damage, such as harm to ships; unpaid debts; treaty violations; and, of course, self-defense. As the seventeenth-century Dutch philosopher and jurist Hugo Grotius—often called “the father of international law”—wrote in Commentary on the Law of Prize and Booty, a “war is said to be ‘just’ if it consists in the execution of a right.”
Because war was conceived as a means of enforcing rights, international law recognized the right of conquest. Land and property could be seized to remedy the wrongs that had triggered the conflict. “In seizing prize or booty,” Grotius explained, states attain “through war that which is rightfully [theirs].” To be sure, powers often claimed what was not rightfully theirs. But because no supreme authority existed to judge the legality of wars, the international system effectively presumed that every conquest was just. Might made right. When the United States launched a war against Mexico in 1846, for example, a main legal justification was Mexico’s unpaid debts. In return for stopping the military campaign, the United States forced Mexico to sign a treaty ceding 525,000 square miles of territory that became the American Southwest in exchange for $15 million and forgiving the debts.
This outcome was far from unique. States often practiced what became known as “gunboat diplomacy”—the use of military threats to advance political or economic demands—to pressure weaker countries into signing unequal treaties. If it was justified for a state to wage war in defense of its rights, then it was justified to threaten war in defense of those rights. In early 1854, U.S. Commodore Matthew Perry exemplified this logic when he sailed into Edo (now Tokyo) Bay with a fleet of American warships. He claimed that the United States had a legal right to trade with Japan, and he made it clear that if Japan did not agree to open its ports, he would do so using military power. The pressure worked: on March 31, 1854, the two countries signed the Treaty of Kanagawa, which opened two Japanese ports to U.S. ships.
For centuries, international law recognized the right of conquest.
Because war was the way in which states pursued their legal rights, waging war was a means of law enforcement, not a crime. When Napoleon lost the War of the Sixth Coalition, in 1814, the European powers that defeated him did not imprison him as a war criminal. Rather, he was sent to the island of Elba, where he was allowed to retain the title of emperor and rule the island as a sovereign. Even after he returned to mainland Europe and was again defeated at the Battle of Waterloo, his subsequent exile to St. Helena in the South Atlantic was not a criminal punishment. It was a preventive measure—a kind of quarantine—meant to keep him from once again unleashing war on Europe.
Not only did states possess the right to conquer other countries’ territory, practice gunboat diplomacy, and enjoy immunity from criminal prosecution for waging war; they were also bound by strict duties of impartiality toward belligerents. Neutral states could not put sanctions on warring parties. If they did, they would be interfering with the belligerents’ efforts to assert their legal rights; if a state violated that duty of neutrality, it created a just cause for war against it. Conquest was lawful, but imposing economic sanctions against belligerents was not.
Under this legal order, which lasted until the early twentieth century, powerful states freely resorted to war to enforce their claims, and weaker states were forced to submit or risk annihilation, yielding a near-constant churn of conflict. With no prohibition on conquest, national borders shifted regularly through violence, and empires expanded by force, entrenching global inequalities. Trade routes were opened and then controlled with cannons, and colonial possessions were won and lost like damages in a lawsuit. The world’s economy remained stunted by the incessant threat of war.
FROM WAR TO PEACE
World War I, however, brought destructive new technologies to the battlefield, and its devastation far outstripped that of previous wars. More than 20 countries eventually entered the fight, and an estimated 20 million people died, around half of them civilians. Once the killing subsided, a desperate search began to find a way to prevent such a catastrophe from happening again. The League of Nations, founded in 1920 to preserve peace through collective security, offered one answer. But the U.S. Senate, wary of being drawn back into European wars, blocked the United States from joining, which hobbled the international organization’s enforcement power.
Around the same time, a new and more audacious idea emerged: to outlaw war altogether. In late 1927, U.S. Secretary of State Frank Kellogg proposed a global treaty formalizing the concept to French Prime Minister Aristide Briand. In less than a year, the so-called Kellogg-Briand Pact of 1928—formally titled the General Treaty for Renunciation of War as an Instrument of National Policy—acquired 58 signatories, the vast majority of states in the world at the time. Establishing the principle that aggressive war was illegal, the parties agreed to “condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another” and pledged to settle any disputes between them “by pacific means.”
Because the pact failed to prevent World War II, it has been widely mocked as naive and ineffective. But in truth, it set in motion a process that gave rise to the modern international legal order. The authors of the pact, for all their ambition, failed to appreciate the scale of what they had done. Once war was outlawed, nearly every aspect of international law had to be reimagined. When Japan invaded Manchuria in 1931, it took U.S. Secretary of State Henry Stimson a year to craft a response consistent with the pact’s principles. Stimson decided that the United States would refuse to recognize Japan’s right to the land it had illegally seized, and the members of the League of Nations soon followed suit. This new principle of nonrecognition, now known as the Stimson Doctrine, became a turning point. Conquest, once lawful, would no longer be recognized. And even if Japan could force China to sign a treaty to give the Japanese the illegally seized land, the agreement would not be recognized as lawful. Gunboat diplomacy would no longer give rise to valid treaty obligations.
After World War I, an audacious idea emerged: to outlaw war altogether.
Although Germany and Japan—both parties to the Kellogg-Briand Pact—flouted it by launching World War II, they ultimately faced its consequences: they lost all the territory they had conquered by force, and their leaders stood trial at war crimes tribunals. The first count in the indictment at the Nuremberg trials charged that “the aggressive war prepared by the Nazi conspirators . . . had been specifically planned in advance, in violation of the terms of the Kellogg-Briand Pact of 1928.”
The pact’s principles also redefined other aspects of international law. U.S. Attorney General Robert Jackson defended the 1941 Lend-Lease Act—which enabled the United States to provide weapons to countries fighting Nazi Germany without a formal declaration of war—by noting that the Kellogg-Briand Pact had altered the laws regulating neutrality. Because the pact’s signatories had agreed “to renounce war as an instrument of policy,” Jackson explained, it followed that “the state which has gone to war in violation of its obligations acquires no right to equality of treatment from other states.” Neutrality no longer demanded that states remain completely impartial in the face of aggression.
Norms, in other words, began shifting in 1928. But the world’s leaders came to realize that ideals were not enough. They needed new legal rules and institutions to give those ideals force. After World War II, the victorious states founded the United Nations to codify the revolution that the Kellogg-Briand Pact had set in motion. In the UNCharter, states are prohibited “from the threat or use of force against the territorial integrity or political independence of any state.” Treaties signed under coercion became formally void, neutrality no longer required impartiality, and leaders who committed aggressive acts of war could be held criminally responsible.
This shift, led by the United States, marked one of the most profound legal transformations in world history. During the nearly eight decades after the UN Charter entered into force, the kinds of interstate wars and territorial conquests that had shaped and reshaped national borders for centuries became rare. Great powers have not openly fought a war against one another since 1945, and no UN member state has permanently ceased to exist as a result of conquest. Conflict, of course, has not disappeared, but it has become far less prevalent. The century that preceded World War II saw over 150 successful territorial conquests; in the decades afterward, there have been fewer than ten.
Some analysts credit the postwar peace to nuclear deterrence, others to the spread of democracy, and others to the rise of global trade. But these interpretations fail to account for the importance of the decision to outlaw war. When the Iraqi leader Saddam Hussein invaded Kuwait in August 1990, violating the UN Charter, for instance, the UNSecurity Council demanded that Iraqi forces withdraw immediately. When they failed to do so, the Security Council authorized other countries to “use all necessary means” to “restore international peace and security.” The United States then led an international military coalition that expelled Iraqi forces from Kuwait. States watching learned that violating the prohibition on the use of force would have consequences. The law shaped states’ behavior not necessarily because they decided that they ought to follow it. It shaped their behavior because it changed how they expected other states—especially the United States—to respond.
Sanctions replaced war as the primary way to enforce international law.
The prohibition on territorial conquest also altered how countries could acquire wealth. Before this rule was established, states’ ability to accumulate wealth often depended on how much territory, resources, and concessions they could capture from other countries. War and conquest were recognized paths to prosperity. By eliminating the right to conquest, the postwar legal order forced states to seek economic growth through peaceful means, primarily trade. The expansion of trade and the prohibition of war went hand in hand, as states could no longer enrich themselves through conquest. Instead, they had to rely on economic cooperation, market competition, and the free flow of goods and capital.
Great powers that had relied on gunboat diplomacy to impose their will, meanwhile, had to substitute checkbook diplomacy. Economic and diplomatic sanctions replaced war as the primary means of enforcing international law. As states became more economically interdependent, they designed increasingly nuanced ways of “outcasting,” or excluding states from the benefits of international cooperation. One such tool, trade sanctions, became a key way that states responded to a wide variety of unlawful actions, such as human rights violations, supporting terrorism, or waging wars of aggression. In 1945, imports and exports accounted for only around ten percent of the world’s GDP; by 2023, they made up 58 percent. Tens of thousands of international organizations emerged, and more than 250,000 treaties were created to help manage this unprecedented level of interdependence. The threat of being excluded from international cooperation became harder and harder to bear.
Thanks to its large share of global GDP and the U.S. dollar’s status as the world’s reserve currency, the United States gained extraordinary power to enforce the rules. For most states, staying on good terms with the United States was a financial imperative. Washington’s role in maintaining the postwar legal order was far from perfect: the United States’ war in Vietnam, its 2003 invasion of Iraq, and its multidecade counterterrorism campaign in the Middle East all relied on overly broad claims of self-defense. But the United States did not violate the essential prohibition on territorial conquest, and it played a critical role in upholding the system, pledging to defend the European states that joined NATO and the American countries in the Rio Treaty, as well as Australia, Japan, New Zealand, the Philippines, South Korea, and Thailand, if any of them faced an illegal attack. Washington’s decision to lead the charge against Iraq’s invasion of Kuwait made it clear that if a state attempted to conquer another, it could well face American-led resistance—even when the United States did not have a treaty obligation to respond. This imperfect but functional system kept major conflicts at bay and ensured that an interconnected world, for all its tensions, did not descend into unchecked violence. States were able to build more prosperous economies without fearing that a greater military power would conquer them or force them into unequal treaties to fork over the spoils.
LEGAL JEOPARDY
That may all be about to change. Prior U.S. administrations can be condemned for their hypocrisy. But the Trump administration’s willingness to give up altogether on the prohibition on war is far more dangerous. The very premise that the United States could acquire Canada, Greenland, or the Panama Canal by force—or that it might claim ownership of Gaza—is not mere realism or a new form of transactional politics based on dealmaking. It is a throwback to an earlier era when might made right. Trump’s rhetoric and actions resuscitate the pre-Kellogg-Briand idea that threatening war or embarking on territorial conquest is a legitimate way to solve disputes and force other states to make concessions.
In addition to threatening conquests of its own, the Trump administration appears poised to give up on defending other states’ right not to be conquered. In April, after threatening to pull U.S. military assistance from Ukraine, Trump warned Ukrainian President Volodymyr Zelensky that if he would not consider a U.S.-brokered peace plan that, according to the Financial Times, could cede 20 percent of Ukraine’s territory to Russia, he would face “losing [his] whole country.” Trump has already brought back gunboat diplomacy by using the threat of force to coerce other countries into signing treaties on his terms; military threats helped obtain concessions from Canada and Mexico. Trump’s tariff policy also undermines the prohibition on conquest by diminishing the power of economic sanctions as a law enforcement tool. Sanctions are most effective if used rarely and in response to clear violations of international law. Slapping tariffs of 25 percent on other countries on a whim, as Trump did to Canada and Mexico, corrodes the impact economic penalties have to punish real unlawful behavior.
Trump made a direct attack on the power of sanctions as an enforcement mechanism when he signed an executive order threatening to sanction judges and lawyers associated with the International Criminal Court. That move turned a tool for enforcing international law into a weapon to undermine it. More broadly, by unraveling states’ interdependence, the isolationist economic policies that Trump is pursuing diminish other states’ ability to punish breaches of international law by outcasting, leaving them with little choice but to resort to military force or to allow violations to go unchecked.
Trump’s various rhetorical salvos and policy shifts may seem chaotic. But they all form part of a wider attempt to dismantle the postwar legal order. This assault is especially dangerous because it is being carried out by the country that built and, albeit imperfectly, maintained that system. Trump may not follow through on all his threats: some may be blocked by the courts or by domestic political opposition, and other leaders may not immediately mimic him. But his threats alone dangerously erode the set of assumptions about behavior, restraint, and consequences that uphold the prohibition on conquest.
Those assumptions—the belief that most states, most of the time, will behave as if the rules matter—allow weaker states to make long-term plans, investors to commit capital across borders, and governments to respond collectively to violations of law. If the world’s most powerful state can flout long-settled expectations with impunity, others are likely to feel they can do the same. And once states no longer expect one another to play by the rules, the system that depended on that expectation will crumble—not all at once, but piece by piece until it collapses altogether.
THE RIGHT FIGHT
If the prohibition on the use of force collapses, Putin, Trump, and Chinese President Xi Jinping may well agree to simply carve the world into spheres of influence. Their countries would then be free to terrorize states within their spheres, extracting concessions from the less powerful in exchange for protection. Although it is possible that such a world would temporarily be relatively quiescent, it would also be far less free. It is more likely that the kinds of incessant conflicts that the prohibition on war banished would return, yielding a world in which, in the famous words of Thucydides, “the strong do what they can and the weak suffer what they must.”
There is another potential path, but it would require courage and quick action. In 2022, 142 countries joined the United States in supporting a UN General Assembly resolution condemning Russia’s attempted annexation of Ukrainian territory as unlawful. Those other states could join forces to reaffirm the prohibition on territorial conquest without relying on the United States as its chief enforcer. There are some signs that Europe intends to step into the gap the United States has left. After the disastrous March meeting in the White House in which Trump and Vice President JD Vance belittled Zelensky and appeared to threaten to abandon Ukraine, Europe rallied to back Ukraine’s right to sovereignty. British Prime Minister Keir Starmer pledged that European countries would increase their military spending and assemble a “coalition of the willing” to defend Ukraine, and the president of the European Commission, Ursula von der Leyen, vowed that the European Union would present a plan to support the country.
But Europe cannot take the United States’ place as the world’s policeman. It cannot muster the necessary military power, economic influence, and political unity. Even if it could, it would be a mistake for the world to overly rely on another actor. Any serious attempt to safeguard the prohibition on the use of force cannot be made without acknowledging the problems with the system that secured it. When the UN was established, five powerful countries—China, France, the Soviet Union, the United Kingdom, and the United States—gave themselves a privileged position as Security Council permanent members with the power to veto any UN enforcement actions. And the United States’ overwhelmingly dominant role in the order meant that when Washington broke the rules—for example, when it invaded Iraq in 2003—nobody was able to hold it to account.
These flaws delegitimized the legal order prohibiting the use of force, particularly in the eyes of states in the global South. This distrust means that some countries may not recognize the value of what they will lose when Trump dismantles that prohibition. Publicly acknowledging the postwar legal order’s weaknesses—and its defenders’ frequent failure to live up to their own ideals—is a crucial first step toward creating a more robust legal order. Maintaining the prohibition on the use of force will require fresh thinking about international institutions: a renewed system for ensuring international peace and security must empower a wider variety of states to share the responsibility of upholding legal norms, making them more legitimate and resilient to domestic shifts in any one country.
Midsize and small countries need to form broad coalitions to defend the prohibition on the use of force. Many analysts assume that the relative peace that has predominated for 80 years could never have been sustained without a primary, powerful state guarantor. But this view underestimates the real power that states can wield when they work in concert. The European Union is an example: none of its 27 member states has great power on its own, but together they are a force.
The UN General Assembly, in which all 193 member states have an equal vote, should play a leading role. The body does not currently have the Security Council’s enforcement powers, but as an organ responsible for maintaining international peace and security, it can assert more power to enforce the charter’s prohibition on the use of force. A recent reform known as the “veto initiative” demonstrates how it can do more. Created after Russia invaded Ukraine, this procedure refers any vetoed Security Council resolution to the General Assembly for debate. General Assembly resolutions passed under this provision afforded states legal backing to coordinate sanctions against Russia and provide Ukraine with arms and financial support. They also led to the creation of an international register of damages to pave the way for postwar reparations.
States should also work within regional or issue-specific coalitions to achieve shared goals. Such coalitions have begun to form: the Council of Europe, for instance, has announced that it is establishing a court to gather evidence against Putin and other Russian leaders and eventually try them for the crime of aggression in Ukraine, and members of the so-called Hague Group—Bolivia, Colombia, Cuba, Honduras, Malaysia, Namibia, Senegal, and South Africa—are working to enforce decisions made by the International Court of Justice and the International Criminal Court regarding the war in Gaza. In May, foreign ministers from the African Union and the European Union vowed to strengthen their partnership on peace, security, and economic matters, offering a potential starting point for a peace coalition that does not rely on the United States.
Trump is shredding what is left of the norm against using force.
Officially, the UN Security Council is currently the only body that can authorize states to go to war to enforce the law. But nothing is stopping countries from creating an “outcasting council,” an organization to authorize joint sanctions against states that violate the prohibition on the use of force or other critical international laws. Sanctions have not always successfully curtailed unlawful behavior, in part because coordinating them on an ad hoc basis is slow and unpredictable. But if states work together consistently to trigger automatic, coordinated responses to certain unlawful acts, they could make this tool far more effective.
Most of all, securing the prohibition on the use of force depends on states recognizing how much good it has enabled, how hard it was to establish, and how much chaos could ensue if it vanishes. If countries respond to the United States’ abandonment of its enforcement role by creating new institutions to take its place, that would send a powerful signal. American leaders may claim that might makes right, but they would be in the minority, and that stance would isolate them. If, for example, Washington were to pursue its threat to seize the Panama Canal, states could coordinate to outcast the United States with economic sanctions and diplomatic penalties or even by withdrawing their permission to allow U.S. bases in their territories. Demonstrating that other countries are willing and able to join together to impose costs on the United States when it breaks the law would help counter the deep damage the Trump administration has done—and would affirm that a wider variety of countries can play a more equal role in shaping and enforcing international law.
Trump’s rise does not constitute the only threat to the prohibition on the use of force. China and Russia are seeking to reshape international norms to suit their interests. But if more states took collective responsibility for enforcing the core rules of the international system, these countries, too, would have to take notice. It is far from clear whether countries such as France, Germany, and the United Kingdom—which are used to dictating the terms of global engagement—will be prepared to share that power. It is also unclear whether countries long excluded from global decision-making can place their trust going forward in an international legal order grounded in the prohibition on the use of force. But supporting such an order is critically important. Playing China, Russia, and the United States off one another might seem to offer short-term advantages to developing countries, but in the long run, these countries risk becoming the spoils of great-power rivalries, with little capacity to direct or control their own futures.
The system that preserved relative peace and prosperity for nearly eight decades is not self-sustaining. It must be vigorously defended. After World War II broke out, U.S. policymakers realized that the failure to establish a durable postwar order following World War I had sown the seeds of future chaos. History’s lesson is that waiting until a moment of crisis has passed to begin planning for what comes next is a recipe for failure. Just as policymakers in the 1940s sought to establish lasting peace out of the disorder of war, today’s leaders must design institutions, alliances, and strategies to secure peace rather than sit and watch as Trump rewinds the clock.
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