In a Tuesday evening address at a US Army base that has 9300 troops in Iraq, Bush acknowledged the toll of the 27-month-old war.
At the same time, he aimed to persuade sceptical Americans that his strategy for victory needed only time to be successful.
"Like most Americans, I see the images of violence and bloodshed. Every picture is horrifying and the suffering is real," Bush said. "It is worth it."
It was a tricky balancing act, thought necessary by White House advisers who have seen persistent attacks eat into Americans' support for the war and for the president, as well as increased discomfort among even Republicans on Capitol Hill.
Bush marked the first anniversary of the transfer of power from the US-led coalition to Iraq's interim government by focusing on progress in the past year and promising success.
"The terrorists can kill the innocent, but they cannot stop the advance of freedom," he said. "They will fail."
He was rejecting calls to set a timetable for withdrawing 135,000 American troops. Instead, he argued for maintaining the present two-pronged strategy: Equipping Iraqi security forces to take over the fight and helping Iraqi political leaders in the transition to a permanent democratic government.
"The work in Iraq is difficult and dangerous," Bush said. "We have more work to do and there will be tough moments that test America's resolve."
Vice-President Dick Cheney had
said Iraq fighting was nearly over
Bush's repeated acknowledgment of death and difficulty come less than a month after Vice-President Dick Cheney proclaimed the armed Iraqi opposition "in the last throes".
Still, the president's overriding message was one of optimism.
"The American people do not falter under threat, and we will not allow our future to be determined by car bombers and assassins," he said.
Democrats and other critics said the country needed more specifics than Bush had been giving.
"We just don't have a clue what the criteria for success is," said Democratic Representative John Murtha, a Vietnam combat veteran.
"People are still willing to give the president time if he would just level with them."
Outside the base, opponents of the war protested.
The liberal group MoveOn.org unveiled television advertisements that call the Iraq war "a quagmire".
"We got in the wrong way. Let's get out the right way," say the ads running in several contested congressional districts.
A recent Associated Press/Ipsos poll found a majority of Americans think the war was a mistake.
Public patience is even being tested in military-friendly North Carolina, where signs along the streets of nearby Fayetteville show steadfast support of the armed forces.
"We told them if they established a government we would back off. They established a government, but we're still there. I feel like the war is doing more harm than good"
26-year-old North Carolinian
In the past year, 100 troops from the several North Carolina bases have died in the war, trailing only the toll from California, according to an Associated Press analysis.
A new statewide poll released on Tuesday showed that, for the first time, more North Carolinians think the war is not worthwhile than think it is.
"We told them if they established a government we would back off," said 26-year-old Carrie Dimmick. "They established a government, but we're still there. I feel like the war is doing more harm than good."