Gen. Dan Caine, chairman of the Joint Chiefs of Staff, warned Trump that if the U.S. attacked, Iran could respond by closing the strait, which connects the Persian Gulf and the Gulf of Oman. About 20% of the world's crude oil passes through Hormuz. Caine said in several briefings that U.S. officials had long believed Iran would deploy mines, drones and missiles to close the world's most vital shipping lane.