Since taking office, Mr. Trump has raced to enact his economic vision, aiming to pair generous tax cuts with sweeping deregulation that he says will expand America’s economy. He has fashioned his steep, worldwide tariffs as a political cudgel that will raise money, encourage more domestic manufacturing and improve U.S. trade relationships.

Trump's threat to impose 50% tariffs on imports from the E.U. is just the latest attack. It comes amid a broader souring of relations and after months of economic sparring.