Self-Defence or Aggression? Anticipatory and Preventive War under International Law

The United Nations Charter does not use “war” as the central legal concept governing force. Instead, it addresses international disputes and prohibits the threat or use of force. The term war nevertheless appears in several treaties of international humanitarian law. In general terms, war is understood as armed conflict between states seeking political, economic or military objectives.

Historically, states frequently used war to settle disputes and formally declared it once the political decision to fight had been made. The UN Charter established a different system. Article 2(4) prohibits the threat or use of force against the territorial integrity or political independence of any state. Limited exceptions include self-defence under Article 51 and force authorised by the Security Council under the collective security system.

International law continues to debate whether anticipatory and preventive uses of force constitute lawful self-defence or disguised aggression.

Preventive war seeks to eliminate a possible threat that may develop in the future even when no attack is imminent. Anticipatory force, by contrast, targets an adversary believed to be preparing an imminent attack that cannot be averted by other means. The immediacy of the threat is therefore central to the legal assessment.

The concept of anticipatory self-defence is historically associated with the nineteenth-century Caroline incident. It helped formulate a strict necessity standard: the need for self-defence must be instant and overwhelming, leaving no choice of means and no moment for deliberation. Any response must also remain proportionate to the threat.

Extending self-defence to threats that are not imminent remains highly contested among states and international-law scholars. As a threat becomes more remote, the use of force resembles preventive war and becomes harder to reconcile with the limits imposed by the UN Charter.

In the Nicaragua case, the International Court of Justice confirmed that conduct by some states is not sufficient by itself to create a binding rule of customary international law. General practice must also be accompanied by a belief that the practice is legally required or permitted.

Although some states have characterised military operations as anticipatory self-defence, repeating that description does not settle their legality or automatically create a new exception to the prohibition on force.

An expansive interpretation of anticipatory self-defence risks undermining the prohibition on the use of force. If an asserted future threat were enough to justify attack, states could resort to force on the basis of unilateral assessments that are difficult to verify.

Broadly legitimising anticipatory and preventive strikes could normalise war as an instrument of policy, weaken collective security and threaten the stability of the international legal order.