A weekend drone attack on Saudi Arabia that cut into global energy supplies and halved the kingdom’s oil production threatened Sunday to fuel a regional crisis, as the U.S. released new evidence to back up its allegation that Iran was responsible for the assault amid heightened tensions over Tehran’s collapsing nuclear deal. President Donald Trump said the U.S. had reason to believe it knew who was behind the attack — his secretary of state had blamed Iran the previous day — and assured his Twitter followers that “we are ... locked and loaded” depending on verification and were waiting to...