War - Jus Ad Bellum: United Nations Charter Of 1945

Chapter 1, Article 2(4)

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.

Chapter 7, Article 51

Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs.

Chapter 7, Article 42

Should the Security Council consider that measures provided for in Article 41 would be inadequate … it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operation by air, sea, or land forces of Members of the United Nations.