Vice President Cheney said separately that it did not matter whether the public supported a continued U.S. presence in Iraq, and likened Bush's leadership to that of Abraham Lincoln during the Civil War.

After a reporter cited polls showing that two-thirds of Americans opposed the Iraq war, Cheney responded: "So?"

"There has in fact been fundamental change and transformation and improvement for the better," he said in an interview in Oman with ABC News.

The confident remarks came on the fifth anniversary of the invasion of Iraq, marking a concerted effort by the administration to highlight progress at a time when most Americans are opposed to the venture.

Protesters urging an end to the war converged yesterday in the nation's capital, as well as elsewhere around the country.

Police arrested more than 30 people who blocked the IRS building in Washington. The demonstrators numbered in the hundreds rather than the thousands organizers had anticipated.

Bush's remarks, delivered to employees at the Pentagon, signaled a revival of the bold and optimistic rhetoric the administration regularly used during the early years of the war. He and his aides had largely abandoned such sweeping declarations of success during the last two years as the carnage on the ground increased and public approval of the war plummeted.

Almost 4,000 U.S. military members have died in Iraq, with more than 29,000 wounded.

Bush said an increase of about 30,000 combat troops in the last year had helped "turn the situation in Iraq around" and made worthwhile the "high cost in lives and treasure." He said he would reject any further troop withdrawals if they threatened security improvements.

His remarks came weeks before a key assessment of the war from Army Gen. David H. Petraeus, top commander of U.S. forces in Iraq, and Ambassador Ryan Crocker. After receiving it, Bush will make a decision on troop levels. He so far has outflanked efforts by Democrats in Congress to force more withdrawals.

The U.S. troop level in Iraq, now at nearly 160,000, is slated to drop to about 140,000 by July, which is near the level of a year ago, when Bush ordered the buildup to tamp down spiraling violence.

Bush took aim at Democrats such as presidential candidates Hillary Rodham Clinton and Barack Obama, both of whom have vowed to quickly withdraw troops, and disputed "exaggerated estimates" of the war's cost. One widely noted calculation by a Nobel Prize-winning economist puts the long-term price tag at $3 trillion or more.

"War critics can no longer credibly argue that we're losing in Iraq, so now they argue the war costs too much," Bush said.

Sen. Charles E. Schumer (D., N.Y.) and Rep. Carolyn Maloney (D., N.Y.) responded by sending Bush a letter asserting that the administration has refused to provide transparent or accurate cost information to Congress.

In one disputed portion of his address, Bush resurrected assertions that Osama bin Laden and his followers have played a central role in the Iraqi conflict. Bush suggested that a backlash among local Sunni Muslims to the group al-Qaeda in Iraq amounted to "the first large-scale Arab uprising against Osama bin Laden, his grim ideology and his terror network."

Many terrorism experts say there are few operational contacts between bin Laden's group and its Iraqi namesake.

Bush made clear he would prosecute the war as he deemed fit until the end of his presidency.

He starkly described the costs of trying to end the war too quickly. From his perspective, retreat would lead to chaos in Iraq, embolden al-Qaeda to pursue an attack on the United States, and encourage Iran to develop nuclear weapons.

"To allow this to happen would be to ignore the lessons of Sept. 11 and make it more likely that America would suffer another attack like the one we experienced that day," Bush said.

Those Who Gave All: See names and photos of U.S. personnel who died during the Iraq war: http://go.philly.

com/war