WASHINGTON—The Trump administration has been steadily ramping up its rhetoric on the threat posed by Iran this year. But the U.S. military has scaled back its presence in the Persian Gulf region, say officials and military experts, removing ships, planes and missiles that would be needed in a major confrontation.

There has been no U.S. aircraft carrier strike group in the Persian Gulf since the Theodore Roosevelt left for the Pacific in March, the longest period of time in two decades that a carrier hasn’t traveled those waters,...