WASHINGTON — One of the Navy’s oldest transport ships, now converted into one of its newest platforms for warfare, arrived in waters off Bahrain late last week, a major addition to the enlarged presence of American forces in the Persian Gulf designed as a counter to Iran.

The keel for the ship, the Ponce, was cast in 1966, and the vessel, nearing the end of its service, was to have been scrapped. But the Ponce was reborn as a floating forward base for staging important military operations across the region — the latest example of the new American way of war.

The first mission of the reborn Ponce was designed to be low profile and defensive, as an operations hub for mine clearing in the Strait of Hormuz, a counter to threats from Tehran to close the vital commercial waterway. In that role, the Ponce will be a launching pad for helicopters, a home to underwater diver teams and a seaborne service station providing fuel and maintenance for minesweeping ships.

But with the relatively simple addition of a modular barracks on the deck, the Ponce can also be a mobile base for several hundred Special Operations forces to carry out missions like hostage rescue, counterterrorism, reconnaissance, sabotage and direct strikes. Even with the addition of the barracks, there is ample room for helicopters and the small, fast boats favored by commandos.