Grave violation of international law
The prohibition on the acquisition of territory by force has underpinned the international order that emerged after the ravages of World War II. The prohibition on annexation is built into the United Nations Charter. The UN Security Council prohibited annexation in UNSC Resolution 242 in November 1967, which has served as the bedrock for peacemaking. Annexation would strengthen the resolve of the International Criminal Court to conduct investigations into alleged war crimes, and could lead to tangible punitive action.
See a detailed position paper here.