"The order will provide the commander-in-chief additional flexibility should military options be required to protect American lives, citizens and interests in Iraq," the Pentagon statement said.
The Bush, accompanied by the guided-missile cruiser Philippine Sea and the guided-missile destroyer Truxtun, was expected to arrive in the gulf Saturday evening.
The Bush had left its home port of Norfolk, Va., in February and had been operating in the Middle East.
Obama ruled out sending American ground troops into Iraq or engaging in any long-term military campaign, saying, "We're not going to allow ourselves to be dragged back in." He warned that U.S. airstrikes would have little lasting effect unless Iraqi leaders eased the country's sectarian divisions.