Article 2(4) of Chapter I of the United Nations Charter, which is in force today: “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”.
Reinforced by United Nations General Assembly Resolution 2625 (aka Friendly Relations Declaration)
As well as the Rome Statute of the ICC:
Annexation by the use of force of the territory of another State or part thereof is an act of aggression
And last but not least
The annexation was declared null and void by United Nations Security Council (UNSC) resolutions 252, 267, 271, 298, 465, 476[25] and 478.[26]
Reinforced by United Nations General Assembly Resolution 2625 (aka Friendly Relations Declaration)
As well as the Rome Statute of the ICC:
And last but not least
Ah, so their war crime was a declaration of war against another UN member?