From Grotius to the UN Charter: The Philosophical and Historical Evolution of Justified Force
Tracing the Intellectual Legacy of Hugo Grotius, Emer de Vattel, Henry Wheaton, and Lassa Oppenheim in the Theory and Politics of War, Self-Defence, and International Order
March 01st, 2026
Berlin Global’s Sunday Article - Philosophy, History and the Problem of Force - The resort to force by states presents one of the most enduring dilemmas in political theory and international relations. From the earliest modern formulations of international law to the present age of nuclear weapons and global institutions, philosophers and jurists have grappled with the justification of war and self-defence.
This article traces the intellectual lineage of key thinkers Hugo Grotius, Emer de Vattel, Henry Wheaton and Lassa Oppenheim whose ideas have shaped the theoretical and legal frameworks governing the use of force. It then considers how these deep traditions inform our understanding of the ongoing conflict involving Iran, Israel and the United States assessing whether contemporary actions align with or depart from long-standing philosophical tenets and legal norms.
Grotius and the Foundations of Just War (1625)
In De Jure Belli ac Pacis (1625), Hugo Grotius laid the foundations for what would become modern international law. Grotius sought to delimit the conditions under which war could be morally justified emphasising that only real and imminent aggression could morally legitimise defensive violence. He rejected wars based on fear of future capabilities alone insisting that the danger must be immediate and unavoidable. His work marked a shift from arbitrary princely violence toward a system in which ethical principles of necessity, proportionality and legitimate cause govern interstate conflict. This natural law approach embedded moral reasoning deeply into the theory of just war setting a standard against purely speculative or preventive violence.
Vattel and the Sovereignty of States
Emer de Vattel’s The Law of Nations (1758) built on Grotius’s groundwork but reflected the rise of sovereign states as central actors in international relations. Vattel emphasised the equality and independence of states and while he retained the idea that force must be necessary and proportionate, his framing was more pragmatic addressing the realities of diplomatic and strategic competition. Vattel recognised that states must protect their security and echoed Grotius’s caution against pre-emptive attacks justified only by fear. For both thinkers, legitimacy in the use of force derives from clear, present danger not from long-term strategic threats.
Nineteenth-Century Refinements: Wheaton and Imminence
By the early nineteenth century, Henry Wheaton in Elements of International Law (1836) translated philosophical principles into criteria that guided actual practice. The diplomatic articulation that emerged in the Caroline affair (1837) that self-defence must be “instant, overwhelming, leaving no choice of means and no moment for deliberation” formalised the concept of imminence. This period demonstrated the convergence of philosophical reasoning and state practice as abstract ethical norms were translated into operational criteria that could govern international disputes.
Oppenheim and Early Twentieth-Century Legal Constraint
Lassa Oppenheim’s International Law: A Treatise (1905) further narrowed acceptable justifications for war stressing that jus ad bellum should be tightly constrained lest states undermine international order. By this period, preventive war directed at hypothetical future threats was viewed with legal and moral scepticism. The interplay between law and morality became more pronounced with doctrine increasingly emphasising restraint, peace and legal consistency.
The UN Charter and the Prohibition of Force
The devastating world wars of the twentieth century led to a structural shift in the international order. The United Nations Charter codified a fundamental prohibition on the use of force permitting it only in self-defence against an armed attack or with Security Council authorisation. This inversion from recognising war as lawful if justified to forbidding war unless exceptions are met derives from the cumulative philosophical movement that sought to limit violence and embed it within legal constraints. Grotius’s core criteria of necessity, proportionality and imminence remain embedded in modern law even as legal structures now operate under precise treaty obligations.
Contemporary Case: The Iran-US-Israel Conflict
The current Middle Eastern conflict involving Iran, Israel and the United States highlights the enduring tension between state security concerns and the legal and moral restraints on force. On 28 February 2026, Israel and the United States launched a coordinated military offensive against targets in Iran, codenamed Operation Lion’s Roar by Israeli authorities and Operation Epic Fury by the United States Department of Defense. This campaign involved extensive air strikes on Iranian military infrastructure, ballistic missile capabilities and leadership targets, and reportedly resulted in the death of Iran’s Supreme Leader Ayatollah Ali Khamenei. The objective articulated by United States and Israeli leaders was to neutralise perceived threats stemming from Iran’s missile programmes and its alleged pursuit of nuclear weapons which they characterised as imminent dangers to regional and global security.
The attacks triggered immediate and widespread retaliation from Iran with ballistic missile and drone strikes directed at Israeli territory and United States forces in the Gulf region. The conflict has spread beyond Iran and Israel drawing in allied militias such as Iran-aligned Hezbollah in Lebanon and affecting multiple Gulf states diplomatically and militarily. Airspace closures and disruption of global supply routes, particularly through the Strait of Hormuz, have added economic and geopolitical dimensions to the crisis. Officials in Washington defended the strikes as necessary to pre-empt threats and prevent Iran from acquiring capabilities that could be used against Israel, the United States or their allies while military leaders framed the campaign as targeted and finite. Nonetheless critics including legal scholars and bodies such as the International Commission of Jurists condemned the operation as a violation of international law arguing that the charter’s strict prohibition on the use of force cannot be circumvented by claims of anticipated future risk in the absence of an armed attack or Security Council mandate.
This episode reflects a central philosophical tension whether a profound strategic threat can legitimately justify force in advance of actual aggression. Grotius and Vattel would likely have emphasised the necessity of imminence rather than speculative danger echoing concerns that preventive war undermines the very international order that law seeks to preserve. Modern doctrine shaped by Wheaton’s and Oppenheim’s refinements and codified in the UN Charter would similarly require unmistakable evidence of an imminent armed attack to legitimise such actions under international law.
Conclusion: Continuity and Challenge in the Theory of Force
The intellectual tradition from Grotius through Vattel, Wheaton and Oppenheim to the UN Charter demonstrates a long-standing effort to reconcile the security needs of states with moral and legal limits on the use of force. These thinkers consistently emphasised that military action must meet strict conditions of necessity, proportionality and immediacy. Grotius insisted that only an imminent and unavoidable threat could justify defensive action. Vattel reinforced this by stressing the importance of respecting the sovereignty of other states. Wheaton translated these ideas into concrete criteria that could guide diplomacy and state practice. Oppenheim narrowed the legal scope of permissible force and highlighted the risks that speculative preventive war posed to international stability.
The UN Charter codified these principles into binding law marking a profound shift from earlier eras in which war was permissible if justified by moral reasoning or political prudence. Today, these centuries of philosophical and legal reflection remain relevant because they offer a framework for evaluating the legitimacy of state action especially in crises that involve complex threats and the potential for large-scale destruction.
The current conflict involving Iran, Israel and the United States illustrates the enduring tension between perceived security threats and the obligations imposed by international law and moral reasoning. Claims of pre-emptive action to neutralise a future threat must be measured against the established principles of imminence, necessity and proportionality. If action is taken on the basis of speculation rather than immediate danger it risks undermining the very international order that these intellectual traditions sought to protect. This tension demonstrates that the lessons of the past are not abstract they remain critical for assessing modern conflicts and guiding state behaviour in a way that preserves peace, legitimacy and the rule of law.
Ultimately, the evolution of thought from Grotius to the UN Charter shows that the moral and legal frameworks governing war are cumulative. They are built on centuries of reflection about the proper limits of force, the duties of states and the consequences of unchecked violence. In applying these lessons to contemporary conflicts, policymakers and scholars are called to weigh not only the immediate tactical gains of military action but also the long-term consequences for international order, regional stability and the moral authority of the state. The principles articulated by these thinkers provide a rigorous lens for evaluating modern disputes and for ensuring that decisions about force are guided by both reason and law.
