Bush boosts troop numbers in Iraq
Mr Bush said there was no magic formula for success in Iraq
Bush statement.
US President George W Bush has ordered more than 20,000 additional troops to Iraq, as part of a new strategy for tackling the conflict there.
In a live televised address, Mr Bush said the deployment would help break the cycle of violence and hasten the day US troops are able to come home.
He said the situation in Iraq was unacceptable, and that responsibility for mistakes rested with him.
However, the announcement brought immediate criticism from Democrats.
Party leaders said Congress would give the proposals the "scrutiny our troops and the American people expect".
"We will demand answers to the tough questions that have not been asked or answered to date," they said in a joint statement.
The Democrats have promised a non-binding vote in both houses of Congress on the strategy.
The US currently has 132,000 troops stationed in Iraq.
Change of strategy
President Bush began with a bleak assessment of the situation, saying that sectarian violence had overwhelmed political gains made by Iraqi since the 2003 invasion.
The BBC's Adam Brookes in Washington said he sounded chastened, saying that any mistakes were his own responsibility.
A change of strategy was needed, he added.
Mr Bush said the vast majority of the new troops would be sent to Baghdad and would fight alongside Iraqi units to secure neighbourhoods from "terrorists and insurgents".
"Our troops will have a well-defined mission, to help Iraqis clear and secure neighbourhoods, to help them protect the local population, and to help ensure that the Iraqi forces left behind are capable of providing the security that Baghdad needs," he said.
But Mr Bush said the effort would succeed where previous operations had failed, because this time troop levels would be sufficient to hold areas that had been cleared.
Another 4,000 troops would go to Anbar province, he said.
Al-Qaeda was planning to seize control of the province, but local tribal leaders were starting to show willingness to fight them, the president said.
"As a result, our commanders believe we have an opportunity to deal a serious blow to the terrorists," he added.
'Beyond military'
Mr Bush also warned the Iraqi government to keep to its commitments.
"A successful strategy for Iraq goes beyond military operations," he said. "Ordinary Iraqi citizens must see that military operations are accompanied by visible improvements in their neighbourhoods and communities."
"So America will hold the Iraqi government to the benchmarks it has announced."
Iraq's territorial integrity also needed defending, Mr Bush said, and this meant interrupting the flow of support for insurgents from Iran and Syria.
Mr Bush summed up by saying the year ahead would require more patience, sacrifice and resolve from the American people.
Victory would not look like those won in previous wars - but failure in Iraq would be a disaster for the United States, he said.
But the president's announcement was sharply criticised by a number of senior Democrats, whose party recently took control of both houses of Congress.
Senator Richard Durbin said the president was ignoring the advice of the former US commander in Iraq, Gen John Abizaid, that increasing troop numbers would prevent the Iraqis from taking more responsibility for their own future.
While most Republican members of Congress backed the president, one senator, George Voinovich, doubted the wisdom of a troop increase.
"At this point I am sceptical that a surge in troops alone will bring an end to sectarian violence and the insurgency that is fomenting instability in Iraq," he said.
Mr Bush is expected to follow his address with a news conference at the White House on Thursday morning, accompanied by Secretary of State Condoleezza Rice, Defence Secretary Robert Gates and Joint Chiefs Chairman Peter Pace.