Five years after he green-lighted the U.S.-led invasion of Iraq, President Bush faced strikingly low approval ratings as he reaffirmed his commitment to "accept no outcome but victory" in the war.

Just 31 percent of Americans approve of how President Bush is handling his job, according to a poll released Wednesday, the anniversary of the start of the conflict in 2003.

Sixty-seven percent of those questioned in a CNN/Opinion Research Corporation survey disapprove of the president's performance.

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The 31 percent approval number is a new low for Bush in CNN polling and is 40 points lower than the president's number at the start of the Iraq war.

"Bush's approval rating five years ago, at the start of the Iraq war, was 71 percent, and that 40-point drop is almost identical to the drop President Lyndon Johnson faced during the Vietnam War," CNN polling director Keating Holland said.

"Johnson's approval rating was 74 percent just before Congress passed the Gulf of Tonkin resolution in 1964, which effectively authorized the Vietnam War. Four years later, his approval was down to 35 percent, a 39-point drop that is statistically identical to what Bush has faced so far over the length of the Iraq war," he said.

But there was no sign that the conflict would end soon.

During a speech at the Pentagon Wednesday, the president called the debate over Iraq "understandable" but insisted that a continued U.S. presence in the region was crucial.

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

"We're helping the people of Iraq establish a democracy in the heart of the Middle East. A free Iraq will fight terrorists instead of harboring them."