Few principles of international law are as clear in theory and as contested in practice as the prohibition on the use of force. The catastrophic experiences of the two World Wars demonstrated the urgent need for a robust legal framework to prevent unilateral military aggression and maintain global peace. In response, the United Nations Charter, adopted in 1945, emerged as a binding international treaty that guides relations among states, ensuring the maintenance of international peace and security. Among its fundamental principles, the Charter explicitly enshrines the non-use of force, particularly in Article 2(4), which states:
“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 principle not only prohibits the use of force but also declares the threat of use of force unlawful, protecting the sovereignty and territorial integrity of states. As Dixon and McCorquodale highlight, the prohibition on force is inseparable from the broader objective of maintaining collective security and preventing the erosion of the international legal order through unilateral military action.
Historical Evolution of the Prohibition on Force
Before the twentieth century, international law largely accepted war as a legitimate instrument of state policy. States could resort to war at their discretion provided certain formalities were observed. There were no comprehensive legal prohibitions against war, and sovereignty was often understood to encompass the right to wage armed conflict.
The Covenant of the League of Nations (1919) represented the first significant attempt to regulate war collectively. While it did not outlaw war entirely, it did impose procedural restraints. Articles 12–15 of the Covenant required member states to submit disputes to arbitration, judicial settlement or inquiry before resorting to war and mandated a cooling-off period before military action. Despite these provisions, the League failed to establish a substantive prohibition on the use of force and lacked effective enforcement mechanisms, leaving states legally able to wage war after fulfilling procedural requirements.
The Kellogg-Briand Pact of 1928 marked the next normative milestone, as states formally renounced war as an instrument of national policy. Although the Pact failed to define war comprehensively or establish enforcement measures, it laid the intellectual foundation for the later absolute prohibition on force enshrined in the UN Charter. Thus, the prohibition on the use of force evolved incrementally, culminating in a legal regime aimed at preventing unilateral aggression and safeguarding international peace.
Exceptions to the Prohibition on Force
Despite the general prohibition, international law does recognize limited exceptions where the use of force may become lawful. The two primary exceptions are self-defence under Article 51 of the UN Charter, and authorization by the UN Security Council under Articles 39-42.
1. Self-Defence under Article 51
Article 51 preserves the inherent right of individual or collective self-defence in response to an armed attack against a Member State. This right is temporary and may be exercised only until the Security Council acts to maintain international peace. Any use of force in self-defence must satisfy the principles of necessity and proportionality and must be reported immediately to the Security Council, ensuring transparency and adherence to international law. The principles of necessity and proportionality in self-defence have deep roots in international law, with key precedents establishing their scope and limits.
The Caroline Case (1837) is widely regarded as the foundational precedent. During a rebellion in Canada, British forces crossed into US territory and destroyed the American steamer Caroline, which had been supplying arms to Canadian insurgents. In response, US Secretary of State Daniel Webster articulated that self-defence is justified only when a threat is “instant, overwhelming and leaving no choice of means, and no moment for deliberation.” Moreover, any defensive action must be strictly limited to what is necessary to repel the threat, establishing the early articulation of the necessity and proportionality principles that still guide international law today.
These principles were further clarified and codified in modern international law by the International Court of Justice (ICJ) in Nicaragua v. United States (1986). The ICJ emphasized that the use of force in self-defence is lawful only in response to an actual armed attack. The Court held that any defensive measures must meet the dual requirements of necessity and proportionality, meaning that force must be limited to what is strictly required to repel the attack and must not exceed the level of threat posed. The ICJ also stressed that self-defence cannot be used as a pretext for unilateral aggression, reinforcing the obligation to respect sovereignty and territorial integrity. Together, the Caroline Case and Nicaragua ruling form the bedrock of customary international law on self-defence. They provide the legal framework for interpreting Article 51 of the UN Charter, ensuring that any use of force in response to an armed attack is measured, necessary and proportionate, and not a justification for unilateral or excessive military action. Historical examples include: Israel during the Six-Day War (1967), claiming anticipatory self-defence; the United Kingdom in the Falklands War (1982) defending its territories against Argentine aggression; and collective self-defence of Kuwait (1990) following Iraq's invasion. These cases illustrate that self-defence is narrowly construed and must be supported by credible evidence and proportional response.
2. Security Council Authorization
The Security Council can authorize the use of force to maintain or restore international peace and security. Under Article 39, the Council determines the existence of a threat, breach or act of aggression. Enforcement measures may then be adopted under Articles 41 and 42, with Article 41 covering non-forceful measures such as sanctions, and Article 42 authorizing armed intervention when necessary. Classic examples include: Korean War (1950) where Resolutions 82 and 83 authorized collective military action against North Korea; and Gulf War (1991) where Resolution 678 empowered member states to use “all necessary means” to restore Kuwait's sovereignty. These mechanisms ensure that the use of force is collective, proportionate and legally justified, distinguishing it from unilateral military aggression.
Contemporary Relevance: India's Actions against Pakistan
The principle of non-use of force remains highly relevant today, with ongoing global conflicts, such as the Ukraine-Russia war and Israel's attacks in the Middle East, underscoring its importance. In this context, the UN report on India's military strikes in Pakistan provides a telling example. Following the April 22 Pahalgam attack, India launched strikes on Pakistani territory in early May 2025. The UN report dated October 16, made public on December 15, described these strikes as an “unlawful use of force,” highlighting violations of the right to life and personal security. Compiled by five UN Special Rapporteurs, the report noted that India had not notified the Security Council under Article 51 and failed to demonstrate the necessity and proportionality required for lawful self-defence, even when targeting non-state actors.
The report also criticized India's unilateral decision to hold the Indus Waters Treaty (IWT) in abeyance, warning that disruption of river flows could severely impact Pakistan's water security, agriculture, livelihoods and fundamental human rights, including rights to food, work, an adequate standard of living and a clean environment. The experts emphasized that such actions violate international law obligations regarding transboundary harm, customary law and erga omnes duties affirmed by the International Court of Justice. Furthermore, the UN report highlighted that India had not provided credible evidence linking Pakistan to the Pahalgam attack. Consequently, the strikes were deemed disproportionate and contrary to Article 2(4), potentially constituting armed attacks against Pakistan and creating a risk of escalation, threatening civilian life in both states. The unilateral holding in abeyance of the IWT was also flagged as potentially violating the rights of Pakistanis dependent on river water and international treaty obligations.
Thus, the prohibition on the use of force under international law remains a foundational principle for global peace and security. While limited exceptions in the form of self-defence under Article 51 and Security Council authorization exist, these are strictly conditional and must be exercised with transparency, necessity and proportionality. The UN report on India's actions against Pakistan illustrates the dangers of unilateral military measures، highlighting both legal and humanitarian implications. By failing to notify the Security Council and undertaking disproportionate military strikes, India's actions appear to contravene the UN Charter and risk escalation. This case reinforces the enduring principle that even in the face of security threats، states must respect sovereignty, uphold international law and protect human rights, ensuring that unilateral actions do not undermine the very framework designed to preserve global peace.
The writer is an LLM graduate in International Law.
