Is the Prohibition on the Use of Force Collapsing?

Introduction

For hundreds of years, war was a lawful means of resolving disputes between states. That began to end with the 1928 Kellogg-Briand Pact, which outlawed war as an instrument of national policy. When the United States and its allies won World War II, they wrote the core principle they had fought for into the new United Nations Charter. Article 2(4) of that document declares: “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.” This prohibition on war has been remarkably effective: Wars of territorial conquest, once common, have become exceedingly rare—and the world has been more peaceful and prosperous than ever before as a result.

That may be coming to an end. Russian President Vladimir Putin appears to be on the cusp of securing a settlement in the Russia-Ukraine war that would effectively cede control over much of the occupied territory seized from Ukraine in an outright act of illegal aggression. China has for years been building military installations on contested rocks, reefs, and islands in the South China Sea, intimidating neighbors and ignoring international legal decisions disputing its claims. Now U.S. President Donald Trump has refused to rule out taking Greenland and the Panama Canal by force, claimed that Canada will become the fifty-first state, and suggested that the United States might own Gaza. With three of the permanent five members of the Security Council seemingly willing to abandon the prohibition on the use of force, and the Council itself mired in dysfunction, what does this mean for the international legal order?

While this is a moment of extraordinary peril, it is also a moment for re-envisioning the world order. After all, for all its successes, the postwar legal order has many faults—not least, casting in amber the power structures that existed at the close of World War II, leaving much of the world underrepresented in today’s global governance structures. If the prohibition on the use of force and the world it created are falling apart, we must not simply mourn the loss. We must consider what we can do to protect against the most catastrophic effects of this shift and seize the opportunity to build a new, more resilient, more equitable international legal order.

While this is a moment of extraordinary peril, it is also a moment for re-envisioning the world order.

On June 23, 2025, the Carnegie Endowment for International Peace hosted a private workshop for a small group of experts and practitioners on these themes. Held under Chatham House rules, the workshop assessed the past, present, and future of the prohibition on the use of force. This article reflects the authors’ own views, informed by the robust conversation among the participants.

How Effective Was the Prohibition on War?

Any conversation about the threat current great power actions may pose to the international legal order requires first evaluating what is at risk. That includes considering how effective the prohibition on war has actually been. On one hand, the legal order has had remarkable success in preventing armed conflict between great powers and delegitimizing territorial conquest and gunboat diplomacy. Yet on the other, the prohibition on the use of force has also been selectively applied, and flagrant instances of military aggression—especially by powerful states and their allies or proxies—have gradually eroded the law’s foundations and global faith in its credibility.

Assessing the effectiveness of the prohibition on war in the post–World War II era is complicated by the reality that the prohibition’s most significant achievements may be what hasn’t happened: Despite numerous military interventions and internationalized civil wars, there has not been another hot war between the world’s most powerful countries. No UN member state has disappeared as a result of conquest, and conquest itself, once common, has become rare. States are rarely forced into unequal treaties on threat of military invasion. Few states attempt to invade their neighbors, and on the rare occasions that they do, they typically face global opprobrium and international sanctions. Indeed, when Saddam Hussein attempted to brazenly seize Kuwait in 1990–1991, a united international community cooperated to overturn that violation.

The fact that states rarely even attempt such actions today makes it easy to forget that they were once quite common. To be sure, this transformation may not be due to the legal prohibition alone—nuclear deterrence and economic interdependence have clearly played a part in great power restraint. But the exercise of such military and economic power has also been structured by law, which has created expectations for acceptable state behavior that have shaped how even the most powerful nations have pursued their interests.

Moreover, measuring effectiveness purely through compliance misses the law’s more complex influence on international relations. Even when states have violated the prohibition on the use of force, they have typically felt compelled to justify their actions within its framework—most commonly in terms of self-defense.  This pattern of justification, while often rightly criticized as mere hypocrisy, demonstrates the law’s continuing influence on state behavior. Yet the use of legal justification in the face of violations corrodes the law’s legitimacy over time. Indeed, Russia’s unconvincing legal arguments for its invasion and attempted annexation of portions of Ukraine have exemplified this dynamic, though recent developments suggest even this pretense of legal compliance may be eroding.

Even when states have violated the prohibition on the use of force, they have typically felt compelled to justify their actions within its framework—most commonly in terms of self-defense.

The prohibition’s effectiveness, moreover, has varied significantly across different dimensions of force and types of war. The norm against territorial conquest has proven highly resilient, with most states accepting the general inviolability of post–World War II borders. The prohibition on gunboat diplomacy—the creation of binding treaties through threats or uses of force—has also proven highly effective. However, some forms of force (Cold War military interventions to topple unfriendly regimes, armed interventions to end civilian suffering, counterterrorism operations against extremist groups operating in fragile states, and surgical strikes to end WMD development and other destabilizing activities of so-called rogue states) have often met with more permissive treatment, creating gaps that powerful states have repeatedly exploited.

Indeed, the law’s implementation has revealed a stark divide between great powers and other states. While smaller nations generally face meaningful constraints on their use of force, major powers, particularly the United States, operate with considerable latitude.  Two prominent cases in point are the NATO intervention in Kosovo in 1999, undertaken without a UN Security Council imprimatur, and the U.S.-led coalition invasion of Iraq in 2003, an action that (notwithstanding U.S. protestations) clearly violated the UN Charter. This double standard has had contradictory effects: undermining the prohibition’s global legitimacy, while at the same time allowing powerful states to reconcile their interests with the framework of international legal limitations on the use of force—perhaps thereby contributing to the prohibition’s longevity.

While smaller nations generally face meaningful constraints on their use of force, major powers, particularly the United States, operate with considerable latitude.

This delicate and tenuous balance is increasingly in doubt, however, as deviations from the prohibition on the use of force have become more egregious. In recent years, the prohibition’s effectiveness has been particularly challenged in three areas. First, the rise of transnational terrorism and nonstate belligerents has created scenarios its drafters never envisioned, leading to contested expansions of self-defense doctrines. Second, the development of new military technologies and hybrid warfare techniques has enabled states to project force in ways that blur traditional legal categories, including definitions of force and aggression. Third, the instrumental use and strategic manipulation of legal arguments to justify military action—as in Kosovo in 1999, Syria since 2014, and Iran in 2025—have gradually undermined the law’s clarity and constraining power.

Recent events suggest that the prohibition now faces its greatest challenges yet. The combination of rising multipolarity, declining American hegemony, and increasingly brazen violations threatens to transform the prohibition on the use of force from a foundational principle of international order into what skeptics always claimed it was: a paper tiger that collapses when confronted with raw power politics.

Yet declaring the prohibition a failure oversimplifies its complex historical legacy. The law helped create an international environment in which military force, while still employed, became increasingly seen as a last resort rather than a routine tool of statecraft. It provided smaller states with legal arguments to resist aggression and established parameters for legitimate use of force that, while often violated, remained meaningful reference points for international society. Last, and most critically, it reinforced national sovereignty and self-determination as a basis for world order by insisting on the rights of all states, large and small, to enjoy territorial integrity, inviolable borders, and a domestic monopoly on the legitimate use of armed force.

What Is the Impact on World Order and the Global Economy if States Can Use Force?

The erosion of international legal constraints on military force threatens to unleash cascading effects that could fundamentally reshape both the global economy and the international order. If major powers come to view threats and the use of force as increasingly viable policy tools, including to wring concessions and seize territory from weaker countries, the carefully constructed post–World War II framework for managing interstate relations—and the global economy—will collapse.  The international order will become more Hobbesian, and any remaining pretense of global collective security will evaporate.

If major powers come to view threats and the use of force as increasingly viable policy tools, the carefully constructed post–World War II framework for managing interstate relations—and the global economy—will collapse.

The economic impacts are already visible. The mere threat of military action creates market uncertainty, disrupts trade flows, and forces businesses to factor growing geopolitical risk into investment decisions. When Iran faces aerial bombardment or Ukraine endures invasion in violation of the UN Charter, the ripple effects extend far beyond their borders, affecting energy prices, shipping routes, and global supply chains. But the deeper consequences lie in how the normalization of force will reshape state behavior and international cooperation. When military coercion becomes an acceptable tool of statecraft, as it was in the era of gunboat diplomacy, it fundamentally alters how states interact across all domains. As trust dissolves, countries become more reluctant to enter into long-term commitments or engage in complex coordination on shared transnational challenges like climate change, fearing that today’s partner could become tomorrow’s adversary. Multilateral frameworks like the UN and the G20 increasingly become venues not for cooperation but for acrimony and confrontation, and governments become even less inclined to contribute to global public goods.

This dynamic creates particular challenges for middle powers and smaller states. Without reliable protection from military force, they must constantly hedge against potential threats, diverting attention and resources from development to defense. The result will be a world where states increasingly prioritize short-term security over long-term growth. The economic costs will compound over time. Markets can price discrete risks, but systemic uncertainty about the basic rules governing interstate relations, including the pacific settlement of disputes, will corrode market confidence and accelerate global economic fragmentation. Companies will reduce cross-border investment, governments will implement protective trade barriers, and states will work to lessen economic interdependence—even when doing so carries significant financial costs. The interwar period provides a devastating example of how the specter of military aggression reinforces autarkical instincts, and, reciprocally, how policies of economic nationalism and discrimination can poison an already toxic geopolitical climate.

Perhaps most worryingly, the erosion of constraints on force, and the decision of great powers to dispense with even the pretense of respecting these prohibitions, threatens to create self-reinforcing dynamics, exacerbating the well-known security dilemma. As more states conclude that international law provides inadequate protection, they will increasingly rely on self-insurance, expanding their own military capabilities and instruments of coercive diplomacy. This will further weaken legal constraints, creating a spiral of declining cooperation and rising militarization, perhaps even spreading nuclearization, as countries seek to deter aggression and reduce their vulnerability to extortion or conquest. Such a transformation will particularly threaten efforts to address global challenges requiring sustained international coordination. Climate change, pandemic response, and the management of outer space and other global commons will all become more difficult in a world where states view each other primarily through the lens of security competition.

As more states conclude that international law provides inadequate protection, they will increasingly rely on self-insurance, expanding their own military capabilities and instruments of coercive diplomacy.

The role of great powers will prove especially crucial. While many celebrate what they see as China’s more responsible—or at least predictable—approach to international order compared to recent U.S. actions, this obscures a more complex reality. China has long engaged in its own forms of coercion, particularly of an economic variety. It, too, has violated the prohibition on the use of force—seizing contested islands, rocks, and reefs in the South China Sea and building military bases on them, despite an arbitral tribunal ruling these actions unlawful. China has also long undermined efforts to sanction states that have violated international law, buying sanctioned oil, gas, and goods that other states refuse to purchase. The key question, then, is not which power will uphold the old order, but what new frameworks might emerge to manage interstate relations as respect for international law erodes.

Traditional policy tools for promoting global stability face mounting challenges. Economic sanctions, long a preferred alternative to military force, increasingly face resistance from states unwilling to sacrifice their economic interests for geopolitical goals. Meanwhile, international institutions designed to manage conflicts of all sorts, such as the UN and the World Trade Organization, struggle to adapt to a world where major powers openly flout their authority. For now, a total breakdown of the global order remains unlikely, since states retain strong practical interests in maintaining basic multilateral frameworks for economic exchange and diplomatic interaction. The question is whether these can be preserved while the prohibition on force—long considered foundational to modern international relations—increasingly erodes.

What Might Come Next?

While wholesale collapse of existing norms remains implausible, several scenarios are taking shape that could define interstate relations in the coming decades.

The first possibility is increasing regionalization, with different parts of the world developing distinct approaches to managing conflict. This is already emerging in Africa, where the African Union and subregional organizations like ECOWAS are taking on peacekeeping and peace enforcement roles previously dominated by former colonial powers, or by the UN. In Asia, ASEAN offers another template for conflict management, one that emphasizes traditional sovereignty and indifference to regime type. This trend toward regionalization could provide new frameworks for legitimizing force, while potentially containing resulting conflicts within geographic bounds. However, such a regionalized security system may threaten the independence of smaller states. Without reliable international protections, they may find themselves increasingly vulnerable to regional hegemons and have little choice but to consent to new regional security arrangements dominated by major regional powers. Indeed, regionalization could permit China, the United States, and Russia to each dominate the less powerful states within its orbit, extracting surplus and exerting unrestrained control.

Second, there may be a rise in “illiberal internationalism,” as authoritarian states and illiberal democracies seek to further transform the international environment in a direction favorable to autocratic oppression and reactionary policies. According to Freedom House, the global democratic recession is now in its nineteenth consecutive year, with major declines, including in the United States. The enemies of democracy may seek to capture existing international institutions or redefine their underlying principles away from human rights and toward regime stability.

Third, more so-called minilateral coalitions may emerge—ad hoc groupings of states cooperating on specific challenges outside of formal, treaty-based international frameworks. This approach may prove more flexible than UN mechanisms, allowing participants to move with dispatch, but it may risk eroding universal norms. It may also weaken existing multilateral organizations whose standing technical capabilities and legitimacy are needed over the long term and erode the capacity to build coalitions across substantive issue areas, including the realization of grand bargains. On the other hand, such variable geometry and à la carte options may allow middle powers and smaller states spanning the Global North and South to more fully participate in the international system, allowing for greater creativity and innovation in solving specific challenges facing states with shared sets of interests.

The United States will play a critical role in deciding which of these paths unfolds. Given America’s long-standing role as the primary champion and enforcer of international law, its retrenchment has created a vacuum that no other power seems fully prepared to fill. Compounding concern, it risks becoming an actor whose own international behavior may need to be policed. Other potential global leaders, meanwhile, suffer from serious limitations. China, while increasingly powerful, shows little appetite for assuming the costly role of global security provider that the United States once played. It lacks the network allies and soft power resources that underpinned American primacy, and it faces significant mistrust in its immediate neighborhood. The European Union, the only other plausible candidate for global leadership, suffers from internal divisions, economic stagnation, a demographic crisis, and a lack of hard power resources.

Looking ahead, the most likely outcome may be a hybrid system of international order, combining elements of regional organization, great power spheres of influence, and issue-specific coalitions.  This could prove more flexible than the current order but also more unpredictable and prone to periodic violence.

The most likely outcome may be a hybrid system of international order, combining elements of regional organization, great power spheres of influence, and issue-specific coalitions.

What Can Be Done?

As the prohibition on force faces unprecedented challenges, a range of practical steps could help preserve core elements of the postwar legal order. Indeed, this moment of extraordinary risk may offer opportunities to rejuvenate the international legal order for a world that looks far different from the world for which it was designed in 1945.

First, international institutions—particularly the UN General Assembly (UNGA)—can play an expanded role in reaffirming and clarifying this foundational principle. For instance, UNGA could help develop clearer parameters concerning the definition and pursuit of self-defense and other contested aspects of the use of force, building on previous efforts like the 1970 Declaration on Friendly Relations. While assembly resolutions are formally recommendations, and thus lack the legal force of law, they can serve as focal points for coordinating state behavior and crystallizing international obligations. Such a role would be consistent with UNGA’s growing assertiveness in matters of international peace and security in response to blockages in the UN Security Council, such as the so-called veto initiative, which directs Security Council members to justify their vetoes to the General Assembly.

Second, states could work to build new coalitions spanning the Global North and South to help sustain basic principles. A coalition of “Friends of the Charter” could unite states committed to peaceful dispute resolution, regardless of geography, political system, or membership in the UN’s regional blocs. But for this to succeed would require moving beyond Western-centric approaches to the international legal order and harnessing and empowering middle powers, established and emerging alike. Middle powers generally have a greater interest in global stability and, unlike great powers, little to gain from undermining the legal order in the short term. The challenge here is to get these states to work together to hold larger and more powerful ones to account.

Third, communication remains an area where much more can be done to repair and reinforce respect for international law. When it comes to the use of force, the states that use force tend to be vocal in issuing their permissive interpretations of the law. States that do not use force, meanwhile, tend to stay silent or speak in ways that receive less attention. Much more could be done to amplify these quieter voices, to make clear that the global community continues to support the core principles of the UN Charter. Public education and engagement also deserve greater emphasis, not least in the United States, whose current administration takes a disdainful attitude toward international law and where public appreciation of its importance is scant.

It is essential to bring home to citizens the value of an effective international legal system to their daily lives, not simply as a bulwark against global chaos but as a structure that advances their daily welfare in innumerable ways.  After all, if people don’t understand international law and its practical value, they are unlikely to be concerned when it is ignored and eroded. Emphasizing the concrete, human consequences of illegal war is likely to prove more compelling than speaking about violations of the UN Charter, for example. Framing international law as a tool for preventing atrocities may resonate more broadly than defending abstract norms. Congressional outreach is also critical. Educational trips for members and their staff, particularly those new to international issues, could help build understanding of international law’s practical importance. Professional organizations like the American Society of International Law and the Carnegie Endowment for International Peace could develop stronger relationships with legislative offices.

It is essential to bring home to citizens the value of an effective international legal system to their daily lives, not simply as a bulwark against global chaos but as a structure that advances their daily welfare in innumerable ways.

Looking ahead, success requires both immediate damage control and longer-term vision. Defending existing institutions remains vital, but space must also be created for reimagining international arrangements that better reflect current power realities while preserving essential constraints on force. This dual approach—simultaneously working within existing structures and exploring new frameworks—demands both pragmatism and creativity from international lawyers and policymakers. It also means engaging seriously with rising powers like China and building broader coalitions that can give voice to previously marginalized perspectives.

Success will require moving beyond nostalgia for American hegemony while preserving core insights about the dangers of unrestricted warfare. States must adapt international law’s tools to contemporary challenges yet maintain its essential purpose: preventing the scourge of war.