WASHINGTON (CNN) -- President Bush marked the fifth anniversary of the Iraq war on Wednesday by calling the debate over the conflict "understandable" but insisting that a continued U.S. presence there is crucial.

President Bush marks the fifth anniversary of the start of the Iraq war with a speech Wednesday at the Pentagon.

"The answers are clear to me," Bush said in a speech at the Pentagon five years from the day the war began in 2003.

"Removing Saddam Hussein from power was the right decision, and this is a fight America can and must win."

Almost 4,000 American troops have died in the war, a painful toll that Bush acknowledged.

"No one would argue that this war has not come at a high cost in lives and treasure, but those costs are necessary when we consider the cost of a strategic victory for our enemies in Iraq," the president said. Watch Bush say how the war must be won »

Bush said victory in Iraq demonstrates American resolve and will prevent attacks on targets in the United States.

"Defeating this enemy in Iraq will make it less likely we will face this enemy here at home," he said.

Bush contends the so-called troop surge he ordered in January 2007 has been a success and was necessary at a point when "the fight in Iraq was faltering."

"The surge has done more than turn the situation in Iraq around; it has opened the door to a major strategic victory in the broader war on terror," he said.

Shock & Awe: 5 Years Later CNN's award-winning reporters are on the ground in Iraq. Anderson Cooper brings you to the frontlines as only CNN can.

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"For the terrorists, Iraq was supposed to be the place where al Qaeda rallied Arab masses to drive America out. Instead, Iraq has become the place where Arabs joined with Americans to drive al Qaeda out." Watch CNN correspondents recall "shock and awe" »

He called Iraq the first place where Muslims have participated in a "large-scale uprising" against Osama bin Laden and al Qaeda.

"We will show that men and women who love liberty can defeat the terrorists," Bush said.

"A free Iraq will fight terrorists rather than harbor them."

Still, large-scale attacks by terrorists and insurgent groups continue in Iraq. Bombings killed six Iraqis and wounded 51 in northeastern Baghdad and Mosul on Tuesday, and the death toll from a Monday suicide bombing in Karbala rose to 50.

Bush said critics of the war -- such as Democratic presidential candidates Sens. Barack Obama and Hillary Clinton -- "can no longer credibly argue that we are losing in Iraq, so now they argue the war costs too much." Watch Obama blame war on Bush, Congress »

Earlier this month, two economists wrote a column suggesting that the war will wind up costing the United States more than $3 trillion.

The March 9 opinion piece in The Washington Post was authored by Joseph E. Stiglitz, a Nobel Prize-winning economist who was chairman of the Council of Economic Advisers under President Clinton, and Linda J. Bilmes, a former chief financial officer at the U.S. Commerce Department.

Senate Majority Leader Harry Reid, D-Nevada, has cited the $3 trillion figure when criticizing the Bush administration's position on the war.

In his speech, Bush called the projected cost "exaggerated."

But he admitted more costs and tough fighting remain, pointing to recent battles against extremists in northern Iraq. But he said any U.S. pullout would put peace at risk in the greater Mideast, emboldening al Qaeda and Iran.

Despite Bush's arguments, the war remains unpopular with Americans.

In a CNN/Opinion Research Corp. poll conducted Friday through Sunday, fewer than one in three respondents -- 32 percent -- said they support the war, while 66 percent said they oppose it. Sixty-one percent of those polled said the next president should remove most U.S. troops from Iraq "within a few months of taking office."

Meanwhile, two of the world's leading humanitarian groups said this week the situation in Iraq leaves little room for optimism.

"Despite claims that the security situation has improved in recent months, the human rights situation is disastrous," Amnesty International says in a report titled "Carnage and Despair: Iraq Five Years On."

And the International Committee of the Red Cross, in a report called "Iraq: No Let-up in the Humanitarian Crisis," writes, "Despite limited improvements in security in some areas, armed violence is still having a disastrous impact." E-mail to a friend

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