A federal judge late Saturday blocked President Donald Trump’s executive order invoking the Alien Enemies Act to deport migrants—even ordering the administration to turn around flights that had already departed—the latest in a string of court rulings against Trump and cost-cutting czar Elon Musk.

President Donald Trump has invoked the Alien Enemies Act for the first time since World War II, granting himself sweeping powers under a centuries-old law to deport people associated with a Venezuelan gang.