Abizaid and Casey have opposed sending more troops to Iraq, and their departures could make it easier for Bush to send more soldiers to the war. One option calls for sending five or more additional combat brigades ¡ª roughly 20,000 or more troops.

Apart from any increase in Iraq, Bush said the military's overall size should be increased to relieve the heavy strain on US troops, reversing the previous position of his administration during Donald Rumsfeld's Pentagon tenure. Bush also said a troop surge in Iraq would have to be for a specific mission.

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His remarks appeared intended to address doubts voiced by prominent military officials who worry that sending more troops to Iraq would be ineffective and put more demands on an already-stretched US military.

"There's got to be a specific mission that can be accomplished with the addition of more troops before, you know, I agree on that strategy," the president said.

The administration says many questions have to be answered about sending in more troops: What would be their purpose, what would they do, how long would they stay and what is the Iraqi government's view on the rules of engagement for more US forces? Also, would the additional troops serve in training positions, in combat, to help civilian forces or for a combination of those roles?

"I'm not going to make predictions about what 2007 will look like in Iraq except that it's going to require difficult choices and additional sacrifices because the enemy is merciless and violent," the president said.

Bush was unwavering about US goals for Iraq.

"Victory in Iraq is achievable," he said. "It hadn't happened nearly as quickly as I hoped it would have. ...

"But I also don't believe most Americans want us just to get out now," the president said. "A lot of Americans understand the consequences of defeat. Retreat would embolden radicals. It would hurt the credibility of the United States."

Bush stepped back from his confident assertion two months ago that "absolutely, we're winning" in Iraq. Wednesday, he said, "We're not winning. We're not losing."

The president said he changed his formulation because "we're not succeeding nearly as fast as I wanted ... and that the conditions are tough in Iraq, particularly in Baghdad."

He said his original remark, on Oct. 25, was made in the spirit that "I believe that we're going to win. I believe that ¡ª and, by the way, if I didn't think that, I wouldn't have our troops there."

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