President Trump’s tariffs are one of the broadest claims of executive power in American history, taxing imports from anywhere on his personal whim. The problem is he doesn’t have that power under the law or the Constitution, as the Court of Appeals for the Federal Circuit ruled late Friday in V.O.S. Selections v. U.S. This is a crucial moment for the Constitution’s separation of powers. A 7-4 majority upheld a lower-court decision striking down the tariffs that Mr. Trump imposed under the 1977 International Emergency Economic Powers Act (IEEPA). In February he invoked the law to slap taxes on imports...