The prohibition on the use of force: what its erosion tells us about the international system
The erosion of the norm prohibiting the use of force is the result of shifting conflict dynamics and a changing international order. With multilateralism in crisis, there are questions as to whether the international legal framework on peace and security may be outdated.
Marked by geopolitical competition, trade wars and the highest number of conflicts since 1945, the current international disorder has sparked a debate over the applicability of and respect for international law both during armed conflict and what justifies waging it. This, in turn, has led to questions over the implications for the international system. Recent outright violations of the ius in bello (i.e., international humanitarian law (IHL) or the law of armed conflict, which disciplines conflict parties’ conduct in war) by Israel, Russia and Sudan’s warring parties, for example, have taken place together with increasingly diverging interpretations of the ius ad bellum (i.e., the law that regulates states’ right to wage war). More recently there have been actions taken outside the law, like the United States’ strikes against Venezuela-based organised criminal groups.
The prohibition on the use of force and its resilience over time
Today, both great and middle powers are challenging the prohibition on the use of force, pushing it to the brink of collapse. A core pillar of the post-Second World War international architecture, the prohibition on the use of force as an instrument of national policy was first introduced in 1928 with the Kellogg–Briand Pact. However, it was short-lived as it was not a universal norm and lacked an enforcing institution like the United Nations Security Council (UNSC). After 1945, to wage war was no longer among states’ rights. The UN Charter outlawed the ‘use of force against the territorial integrity or political independence of any State’, including the threat of such use (Article 2(4)), with the exclusion of self-defence (Article 51).
Overall, for nearly 80 years, the prohibition on the use of force has been exceptionally effective, given that no inter-state war between great powers has occurred. Furthermore, the number of sovereign states more than tripled as small states became less subject to rule or untoward influence from powerful ones. The UN Charter’s recognition of the right to self-determination, which partially ushered in decolonisation, also helped achieve this outcome. Banning the use of force was thus instrumental to a new era of cooperation, trade and interdependence that led to unprecedented levels of global prosperity – albeit highly uneven ones.
Certainly, there have been repeated strains to the system and controversial cases of the use of force. During the Cold War, the two superpowers de facto occasionally breached the prohibition on the use of force against weaker states, both covertly (through supporting military coups, for example) and openly (most notably, the US in Cambodia and the Soviet Union in Afghanistan), while also engaging in proxy wars (i.e., providing material and advisory support to one side of conflicts within weaker states).
Shifting conditions and the progressive erosion of the law
After the Cold War, the gap between the conditions surrounding the peace and security norms adopted in 1945 and the new realities of the twenty-first century (i.e., shifting geopolitics, emerging non-state actors and novel security threats) progressively widened. Security threats that were either minor or less relevant in 1945 (such as terrorism and humanitarian emergencies) were addressed through the adoption of new principles and approaches, including humanitarian intervention and the responsibility to protect. The legal responses to the rise of international terrorism after 9/11 were attempts by some states to broaden the parameters of self-defence (i.e., the Global War on Terrorism). Some of these legal justifications were controversial, though. More recent factors that challenge the boundaries of the ‘use of force’ concern the increasing use of cyber warfare and drones, which have considerably widened the definition of ‘battlefield’.
Cumulative challenges to the prohibition ultimately took a toll – especially when they occurred alongside the erosion of other international norms (e.g., IHL) and when the great powers were responsible for their violation. The 2003 US-led invasion of Iraq arguably stretched the definition of self-defence as the United Kingdom and the US claimed to be enforcing previous UNSC resolutions and the right to collective pre-emptive self-defence. This was perceived as a defining moment of this erosion. Not only because the US had been one of the main forces leading the post-1945 normative system and its main security guarantor, but also because new threats emanating from non-state armed groups (NSAGs) had led to controversial interpretations of the exercise of self-defence.
Increasingly, divergent interpretations and selective applications of the law led to perceptions of double standards and undermined its legitimacy, prompting other powers to follow suit. In the last decade, China’s military build-up in contested areas of the South China Sea has been widely seen as contravening the ban on the use of force. The most egregious violation of the ban took place in 2022 with Russia’s unprovoked full-scale invasion of Ukraine. More recently, US President Donald Trump’s second administration has raised the prospect of using force against Canada and Panama, as it has threatened to do so against Venezuela. That threats of the use of force come from the US is even more concerning, as it lowers the bar of what is considered acceptable in state relations.
The crisis of multilateralism
Trump’s second administration has also undermined the UN system and multilateralism in conflict resolution, in turn eroding the UN’s legitimacy and fuelling a lack of confidence in international law. Concurrently, another factor driving the erosion of norms is the UNSC’s recent inability to act. As it has been argued, because it is not a representative body, the Council’s legitimacy stems from its ability to mitigate, de-escalate or resolve crises. Since 1945, this has been the case in countless cases of both intra-state and inter-state conflicts. In civil wars, there is a consensus on the positive impact of peacekeeping operations in saving lives, preventing escalation and supporting peaceful transitions – with tragic exceptions like Bosnia, Rwanda, Somalia and Sudan. Regarding inter-state conflicts, the Council has resolved nearly 70% of the conflicts or disputes brought to it between 1945 and 2023.
However, peacekeeping was already in crisis even before the Trump administration started to cherry-pick UN functions and to slash funding. As for inter-state disputes, 2024 witnessed the highest number of inter-state wars (four) since 1987, while in 2025, new military confrontations broke out between Afghanistan and Pakistan, Iran and Israel, Cambodia and Thailand, and India and Pakistan, among others.
Has the current legal framework become outdated?
One of the main issues is that states have been collectively unable to frame security breaches outside the right of self-defence from a legal standpoint. As battlefields have widened to include novel elements (e.g., drones, cyber warfare, information technology, fragmented NSAGs), states have not established norms to regulate when and if any of these should be considered security breaches.
Conflicts have substantially changed: the global proliferation of NSAGs over the last 15 years, which has underpinned an escalation of intra-state armed conflicts (i.e., civil wars), has not been met with meaningful responses from a legal point of view. As NSAGs have often been defined as ‘enemy combatants’, this has led to a blurred distinction between combatants and civilians – a fact that has negatively impacted the application of IHL, for example.
Ultimately, there is a political and strategic argument for improving and strengthening international law, particularly the prohibition on the use of force. The latter is respected as long as it aligns with the interests of the great powers. In the last decade, shifting balances of power (both between and within states) and technological advancements across every possible domain have underpinned a more chaotic and unpredictable international system. States are the engine of these changes as they re-orientate their strategic priorities exclusively toward self-interest, while sidelining collective security and global public goods.
Therefore, international institutions and norms risk becoming obsolete, yet they are still crucially relevant. After the Second World War, their number boomed across every domain, from trade and economic development to scientific research to arms control, among others, shaping regulations in countless social, cultural and economic domains conducive to peace. Because norms remain relevant, opportunities to revise the ban on the use of force exist. The fact that states go a long way to legally argue that their actions are either within the parameters of the law or in line with its spirit (for example, Israel’s operations in Gaza) is proof that both the laws regulating state’s right to wage war and IHL are still relevant today.
However, more recently, there have been signs of states going in the opposite direction: after the US has specifically disavowed any attempt to legally justify its actions against Venezuela or attacks on vessels suspected of drugs trafficking, Israel has also publicly used legal defences less and less with respect to its actions in Gaza. Adherence to the norm derives from the shared expectation among UN Member States that the law will be respected and that transgressors will be punished. Currently, that expectation has decisively weakened, but not completely: for example, Russia’s attack on Ukraine has had significant political and economic repercussions for Moscow, particularly regarding its participation in the international system.
Working on tangible incentives for states to adhere to norms may reconcile a state’s strategic interest with international respectability. A final silver lining concerns economic interdependence, which remains considerable in both scope and depth despite ongoing trade wars and inter-state conflicts. This interdependence compels states to continuously reassess their actions and posture, making the (re)discovery of shared strategic interests and global public goods a plausible scenario going forward.
This article is part of an ongoing roundtable discussions series on Armed Conflict Dynamics and International Disorder, hosted by the Conflict, Security and Development Programme (CSDP) and IISS–Americas.