A state may lawfully resort to unilateral use of force outside of its territory in the following circumstances:
1) When it has been subjected to an armed attack on its territory, vessels or military forces;
2) When the imminence of an attack is so clear and the danger so great that the necessity of self-defense "is instant (and) overwhelming;"
3) When another state that has been subjected to an unlawful armed attack by a third state requests armed assistance in repelling that attack;
4) When a third state has unlawfully intervened with armed force on one side of an internal conflict and the other side has requested counter intervention in response to the illegal intervention; or
5) When its nationals in a foreign country are in imminent peril of death or grave injury and the territorial sovereign is unable or unwilling to protect them.
The Lawful Resort to Unilateral Use of Force,
Yale J. Int'l L.
Available at: https://digitalcommons.law.yale.edu/yjil/vol10/iss2/7