The present article analyses threats of armed force by states against other states. After determining what conduct amounts to a ‘threat of force’, the current status of its prohibition is investigated. In particular, this article contends that the prohibition of the threat of force contained in Article 2(4) of the UN Charter reflects customary international law. The fact that only a few states have supported the legality of threats for certain law enforcement purposes prevents the prohibition of any threats of force from being qualified as jus cogens. This status should be limited to the prohibition of threats of aggression, which constitutes the common denominator of the practice and opinio juris of all states.

The consequences of threats of force under the law of treaties, the law of state responsibility and international criminal law are then discussed, in particular treaties, the conclusion of which has been obtained by the threat of force in violation of the UN Charter are void ab initio. Article 41 of the ILC Articles on State Responsibility could also entail a duty of non-recognition of situations procured by the threat of aggression. On the other hand, threats of aggression (let alone threats of less serious forms of the use of force) cannot be qualified as international crimes entailing individual responsibility.

As to remedies at the disposal of the victim state, armed responses to threats of force would be lawful only within the limits of self-defence, and only if the threatened armed attack is imminent enough to meet the Caroline requirements. On the contrary, coercive responses to non-imminent armed attacks are still prohibited under contemporary international law.