The Pentagon has quietly shifted combat troops and warships to the Middle East after the top American commander in the region warned that he needed additional forces to deal with Iran and other potential threats, U.S. officials said. Marine Corps Gen. James Mattis, who heads U.S. Central Command, won White House approval for the deployments late last year after talks with the government in Baghdad broke down over keeping U.S. troops in Iraq, but the extent of the Pentagon moves is only now becoming clear. The Pentagon has stationed nearly 15,000 troops in Kuwait, adding to a small contingent already there. The new units include two Army infantry brigades and a helicopter unit - a substantial increase in combat power after nearly a decade in which Kuwait chiefly served as a staging area for supplies and personnel heading to Iraq. The Pentagon also has decided to keep two aircraft carriers and their strike groups in the region. Earlier this week, the American carrier Carl Vinson joined the carrier Stennis in the Arabian Sea, giving commanders major naval and air assets in case Iran carries out its recent threats to close the Strait of Hormuz, a strategic chokepoint in the Persian Gulf, where one-fifth of the world's oil shipments passes.