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.
[1] However, some states were concerned that use of the veto power by one of the Council's permanent members might prevent that body from taking necessary action, and they insisted upon inserting into the Charter an explicit right of self defense.
[4] Another view is that Article 51 acknowledges the previously existing customary international law right and then proceeds to lay down procedures for the specific situation when an armed attack does occur.
[7] As noted above, imminent threat is a standard criterion in international law, developed by Daniel Webster as he litigated the Caroline affair.
The Caroline affair has been used to establish the principle of "anticipatory self-defense" and is also now invoked frequently in the course of the dispute around preemptive strike (or preemption doctrine).