Art. 2 (4) U.N. Charter"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."
Charter of the United Nations (done at San Francisco, United States, on 26 June 1945) (United Nations [UN]) 892 UNTS 119, 59 Stat 1031, TS 993, 3 Bevans 1153, 145 BSP 805, Ch.I Purposes and Principles, Art.2(4)