It’s hard to tell how serious President Trump is about his proposal to take over and redevelop the Gaza Strip. Last week he posted on social media a zany AI-generated video featuring fast-frame images of a Trump-Gaza skyscraper, paper money raining down on dancing children, and Mr. Trump and Benjamin Netanyahu reclining shirtless on beach chairs. The president is successfully trolling European and United Nations officials, who have insisted the plan would violate international law.

They’re wrong. The legal basis for the proposal is straightforward. Gaza is one of the very few pieces of land not under the sovereignty of any nation, a status known as terra nullius in international law. Such situations are rare because in the postwar era local populations can win recognition for a new sovereign state with relative ease. Once established, sovereignty is hard to extinguish.