Almost 4,000 U.S. troops killed. Nearly 30,000 wounded. Half a trillion dollars spent and counting. | INTERACTIVE: Timeline of Iraq war | AUDIO SLIDESHOW: Touched by war | VIDEO: Protests in D.C.

As the Iraq war entered its sixth year Wednesday, President Bush called the toll "a high cost in lives and treasure.'' Then he called it something else: necessary.

Marking the anniversary of the conflict that has dominated his presidency, Bush addressed the war in a speech at the Pentagon. He defended the war with no doubts. He conceded that it has been harder, longer and more expensive than anticipated but insisted it has all been needed to keep Americans safe.

Protesters demonstrated near the White House and in other cities, including Chicago. A candlelight vigil was conducted in Normal.

Bush, in the speech at the Pentagon, offered some of his boldest assessments of progress and said the war's legacy is absolute: "The world is better, and the United States of America is safer.''

A war-weary country isn't nearly so convinced.

The majority of people think the invasion was a mistake, polls show. However, Americans are more split about how the war is going and when U.S. troops should be pulled home, as reduced violence in Iraq has begun to influence the public view.

"Five years into this battle, there is an understandable debate over whether the war was worth fighting, whether the fight is worth winning, and whether we can win it,'' Bush said. "The answers are clear to me: removing Saddam Hussein from power was the right decision, and this is a fight America can and must win.''

The U.S. has about 158,000 troops in Iraq, and that number is expected to drop to 140,000 by summer. But Bush signaled anew that he will not pull more troops home as long as his commanders worry it will imperil improved conditions in Iraq.

"Having come so far, and achieved so much, we're not going to let this happen,'' Bush said.

Demonstrators converged in the nation's capital, in other big cities like Miami and San Francisco, and in smaller towns in Vermont and Ohio to urge an end to the war. Police arrested more than 30 people who blocked the Internal Revenue Service building in Washington, D.C., and protesters blocked downtown intersections several times.

However, the demonstrators numbered in the hundreds rather than the thousands organizers had hoped for.

But in Chicago, police say about 2,200 people marched downtown to mark the war's anniversary and its continuation.

Chicago police say no one was arrested in Wednesday's demonstration, which began in Federal Plaza around 6 p.m. and caused rolling street closures.

Participants chanted, held signs and linked arms during the march.

Earlier, students held peaceful demonstrations at Loyola University. The group posted anti-war chalk messages around the school and posted signs inside classrooms.

They also hid some 500 toy soldiers around the campus on Chicago's North Side.

In Springfield during a press conference at the Illinois Statehouse, U.S. Rep. Phil Hare of Moline said the president has lost sight of all domestic priorities in exchange for his obsession with victory in Iraq.

In the campaign to replace Bush, Democratic Sens. Barack Obama and Hillary Rodham Clinton pledged to end the war, but squabbled Wednesday over who could do it best.

Democrats in Congress assailed Bush for failed, tired leadership and questioned why he did not push Iraq's leaders to live up to promises.

"All the president seems able to offer Americans is more of the same perpetual disregard for the costs and consequences of stubbornly staying the course in Iraq,'' said Senate Majority Leader Harry Reid.

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

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

Osama bin Laden's al-Qaida network orchestrated the Sept. 11, 2001 attacks. The bipartisan Sept. 11 commission found no collaborative relationship between al-Qaida and Saddam, who ruled Iraq at the start of the war and was later captured, tried and hanged.

Al-Qaida in Iraq, an insurgent group, has grown into a more potent, dangerous organization since the start of the war. Bush speaks of undeniable links between that group and the broader al-Qaida network, although experts question that extent of those ties.

Bush praised Sunni tribal leaders for rising up against al-Qaida in Iraq. He said that has led to similar uprising across the country. Bush put the figure at 90,000 local citizens who are protecting their communities against extremists.

All that, combined with a strategic influx of U.S. troops last year, has "opened the door to a major victory in the broader war on terror,'' Bush said.

"Iraq was supposed to be the place where al-Qaida rallied Arab masses to drive America out,'' Bush said. "Instead, Iraq has become the place where Arabs joined with Americans to drive al-Qaida out. In Iraq, we are witnessing the first large-scale Arab uprising against Osama bin Laden, his grim ideology.''

Bush did not mention that weapons of mass destruction in Iraq - a main justification for the war - were never found.

"This isn't the war we signed up for,'' said Jon Alterman, head of the Middle East program at the Center for Strategic and International Studies in Washington. Back in 2003, he said, Americans expected a quick, decisive defeat of Saddam and no lingering presence.

"What we are seeing is a much more difficult, muddled, nuanced presence at a much higher cost and for a longer time than anyone bargained for,'' he said.

Bush spoke of Saddam's removal as a worthy end in itself, ridding Iraq of death squads, torture chambers and rape rooms.

He also jabbed at political critics who he said "still call for retreat.'' The message was similar to the "no surrender'' theme of Bush's would-be Republican successor, Sen. John McCain, who also warns about pulling troops home too fast.

Nobel Prize-winning economist Joseph E. Stiglitz and Harvard University public finance expert Linda Bilmes have estimated the eventual cost of the war could be as much as $3 trillion when all the expenses are calculated.

Without naming anyone, Bush decried those who he said have exaggerated the expense.

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

In Congress, the leaders of the Joint Economic Committee, Sens. Charles Schumer, D-N.Y., and Rep. Carolyn Maloney, D-N.Y., wrote him and asked for full disclosure.

"The American people deserve a full accounting of what the war has cost in terms of lives, our reputation abroad, and our economy,'' they wrote.