Gerald Herbert, Associated Press
President Bush says Wednesday at the Pentagon that a precipitous U.S. withdrawal would cause chaos in Iraq.

WASHINGTON — Five years from the day he ordered the invasion of Iraq, President Bush acknowledged Wednesday that the war has been "longer and harder and more costly than we anticipated." He warned that "tough fighting" remains and said U.S. troops will not be withdrawn precipitously.

As protesters demonstrated nationwide against a war that has cost nearly 4,000 American lives, Bush told a Pentagon audience of mostly military personnel that the United States cannot retreat. Some in the audience had participated in the invasion on March 19, 2003.

"So long as terrorist danger remains, the United States of America will continue to fight the enemy wherever it makes its stand," Bush said to loud applause.

The address came only slightly less than five years after the president declared major combat operations in Iraq over — a "Mission Accomplished" photo opportunity that proved premature. Today, the president faces deep divisions in Congress. The two remaining Democratic presidential candidates have called for a gradual withdrawal of troops. Largely because of the war, two-thirds of Americans now have unfavorable opinions of Bush.

En route to the Pentagon, Bush's motorcade passed anti-war demonstrators on Pennsylvania Avenue. They held large white banners that read, "How much longer?"

As he has done in the past, Bush defended the Iraq war, saying that defeating terrorists there "will make it less likely that we'll face the enemy here at home."

"Five years ago tonight, I promised the American people that in the struggle ahead we will accept no outcome but victory," Bush said. "Today ... I reaffirm the commitment."

The president defended his decision to go to war, calling Saddam Hussein's government "a regime that threatened free nations." He did not mention the initial rationale for the preemptive strike — that Saddam was hiding weapons of mass destruction. Those weapons never were found.

The "surge" of American troops he ordered a year ago is working, Bush said. "The level of violence is significantly down, civilian deaths are down, sectarian killings are down, attacks on American forces are down. We have captured or killed thousands of extremists. -->"

A precipitous withdrawal, Bush said, would cause Iraq to descend into chaos. Enemies of the United States would see it as "evidence of weakness and a lack of resolve."