Surrounded by Democratic and Republican lawmakers at the White House, the president has signed a congressional resolution giving him the power to wage war against Iraq.
In remarks before the signing, Bush said he hopes military action won't be needed, but indicated he won't hesitate to order it.
Bush said the goal is to "fully and finally eliminate a real threat to world peace and to America."
The president also used the ceremony to further push the United Nations to adopt a new U.S.-backed resolution that toughens weapons inspections and gives the green light to military force if Saddam Hussein doesn't go along.
Bush said it's time for the UN to live up to its purpose, which he said is "to protect our common security."
He also sent a message to world leaders who remain wary of U.S. threats against Iraq, saying "those who choose to live in denial may eventually be forced to live in fear."