The law on the use of force concerns the lawful preconditions for the use of armed (military) force by States and the UN through the SC. This is regulated in Arts 2(4) and 51 and Chapter VII UN Charter.

The UN is the only international body with the power to authorize the use of force, outside cases of self-defence. The UN does not possess its own armed forces and necessarily relies on coalitions willing to contribute their forces.

Collective security concerns the power of the SC to authorize the use of armed force against States or non-State entities, such as pirates.

States are only entitled to use armed force in situations of self-defence. Self-defence arises only where the target State is under an ‘armed attack’. An armed attack is a very significant use of armed force against the target nation.

Whereas an ‘armed attack’ is a necessary precondition for self-defence, it is not a precondition in the context of collective security.

The legality of some types of force is not clear. This is particularly true in respect of humanitarian intervention, pre-emptive self-defence, anticipatory self-defence, and extraterritorial action to save nationals abroad.