Israel is legally the "occupying power" which is a legal position under the laws of war. Specifically The Hague Convention 1907 and the Geneva Convention 1949.
Israel is essentially running a military occupation over the Territories. The Territories were seized from Jordanian sovereignty which were seized from the expiration of the British Mandate over Palestine which were seized from the Turkish Ottoman Empire which were seized from the Christian Crusader Kingdoms.
However, the most relevant laws would be the League of Nations then UN Mandate to the British over Palestine which had set aside the territories in question for the Arab state (Palestine) to be partitioned from the Jewish Israeli State and subject to self-determination as part of the decolonization policy of the UN.