President George Bush last night defied American public opinion, the Democratic-controlled Congress and many of his own generals by sending 21,500 more troops and $1 billion in additional aid to Iraq.
In a 20-minute televised address to the nation the president acknowledged that it was a mistake not to send more troops earlier in the war but argued that success in Iraq was too important for the US to start withdrawing now.
"If we increase our support at this crucial moment, and help the Iraqis break the current cycle of violence, we can hasten the day our troops begin coming home," Mr Bush said.
The first of five new combat brigades will be deployed to Iraq next Monday, with additional brigades arriving at monthly intervals until the summer. A force of 4,000 marines will be sent to Anbar province in western Iraq to counter the Sunni insurgency there.
The deployments are part of a dramatic shift in US military strategy in Iraq that will change the focus from training Iraqi forces to providing security for civilians, especially in Baghdad.
"Our past efforts to secure Baghdad failed for two principal reasons: There were not enough Iraqi and American troops to secure neighbourhoods that had been cleared of terrorists and insurgents. And there were too many restrictions on the troops we did have," Mr Bush said.
The military operation will be accompanied by other counter-insurgency measures, including job creation schemes and other economic measures designed to boost public confidence in the government of Iraqi prime minister Nouri al-Maliki.
The president stressed that Iraqi forces would take the lead in the new military operations and that US support would be linked to political moves to promote reconciliation between Iraq's Sunni, Shia and Kurdish communities.
"I have made it clear to the prime minister and Iraq's other leaders that America's commitment is not open-ended. If the Iraqi government does not follow through on its promises, it will lose the support of the American people - and it will lose the support of the Iraqi people," he said.
The White House believes that stability in Baghdad can be achieved by the summer, so that US troops will be able to pull back to areas outside the capital and that Iraqis will be in control of security in all 18 of Iraq's provinces by November.
The president's speech last night follows almost three months of consultations after last November's congressional elections, which Democrats interpreted as a rejection of the Iraq war. Mr Bush has rejected the two central proposals of the bipartisan Iraq Study Group, which called for the withdrawal of most US combat troops by early 2008 and direct talks with Iran and Syria about Iraq's future.
Democrats in both houses of Congress will next week introduce non-binding resolutions rejecting the troop increase but are unlikely to refuse to fund the president's plan. The new troop deployments will cost $5.6 billion and the economic measures almost $1 billion.