President George W Bush has said he will not accept "anything less than complete victory" in Iraq.
Mr Bush has never laid out his plans in this way before
In a major policy speech, Mr Bush refused to set an "artificial deadline" to withdraw US troops, saying it was "not a plan for victory".
It comes after the release of the first Iraq strategy document, which rejects widespread calls for a timetable.
Mr Bush has come under growing pressure from Democrats on Iraq. Polls give him the lowest approval of his presidency.
They also suggest that six out of 10 Americans think the war in Iraq is not worth the cost.
As such, this was a speech from a president in deep trouble, says the BBC's Justin Webb in Washington.
Harry Reid, the Democratic leader in the Senate, said Mr Bush's speech was "recycled... tired rhetoric", and that the president had "once again missed an opportunity to lay out a real strategy for success in Iraq that will bring our troops safely home".
Speaking at the US Naval Academy in Annapolis, Maryland, Mr Bush said there would be violence in Iraq "for many years" and that US troops would only be able to withdraw as local forces gained competence.
"These decisions about troop levels will be driven by the conditions on the ground in Iraq and the good judgement of our commanders, not by artificial timetables set by politicians in Washington," he said.
Mr Bush said victory would come "when the terrorists and Saddamists can no longer threaten Iraq's democracy, when the Iraqi security forces can provide for the safety of their own citizens, and when Iraq is not a safe-haven for terrorists to plot new attacks on our nation".
This was a partial redefinition of what victory might be, and potentially highly significant, our correspondent says.
Mr Bush also openly acknowledged that there had been "some setbacks in standing up a capable Iraqi security force, and their performance is still uneven in some areas".
But Iraqi forces were regaining control of the country and training programmes had been improved, he said.
Withdrawing US troops before they had accomplished their mission would send the wrong message to the insurgents, Mr Bush added.
"America will not run in the face of car bombers and assassins so long as I am your commander-in-chief," he said.
The new National Strategy for Victory, released hours before Mr Bush's address, defines who the US sees as the enemy in Iraq, listing three groups in declining order of size:
- "Rejectionists" - primarily Sunni Arabs who fared well under Saddam but have lost influence and authority. The US says their resistance will fade if a new democratic government protects minority rights
- "Saddamists" who were active members of the former regime. The US expects their power to wane to the point where Iraqi security forces can defeat them
- "Terrorists" associated with al-Qaeda who want to establish a totalitarian Islamic empire, and who must be killed or captured through counter-terror operations.
"No war has ever been won on a timetable - and neither will this one," the document adds.
There are currently more than 150,000 US troops in the country. The White House has said it expects conditions will permit a reduction in US troop numbers next year, after Iraqi parliamentary elections in mid-December.
The US is spending about $6bn a month to keep its forces in Iraq. About 2,100 Americans have been killed since the March 2003 invasion.