US-Israel Conflict with Iran: Who decides legality?

As the US-Israel war against Iran intensifies with no clear end in sight, a critical question remains: Is this war legal? On March 10, the United States submitted a letter to the United Nations Security Council, justifying its strikes - carried out alongside Israel - as "necessary and proportionate" self-defense under Article 51 of the UN Charter. But does this argument truly meet the legal threshold for self-defense? What rules of international law apply as this conflict escalates?