President Bush, acknowledging mistakes in force levels and war strategy, has ordered an additional 21,500 troops into Iraq to bolster security and support rebuilding in Baghdad and Anbar province.

In a Wednesday night speech from the White House library, Bush pushed forward with a troop increase at a time when many in Congress and the country have been calling for a reduction. The consequences of failure in Iraq, which he equated with a withdrawal now, would only embolden extremists and regional powers such as Iran, Bush said.

“The situation in Iraq is unacceptable to the American people and it is unacceptable to me,” Bush said. “Our troops in Iraq have fought bravely. They have done everything we have asked them to do. Where mistakes have been made, the responsibility rests with me.”

Alternately dubbed a “surge” or an “escalation” — depending on who was doing the talking — Bush’s plan also calls for the Iraqi government to commit to a $10 billion rebuilding effort and more troops and police.

But, he said, in order for the Iraqis to succeed, an increase in American force levels was needed. While Bush said he has told the Iraqi government that the U.S. commitment is “not open-ended,” there was no indication in Bush’s speech that the 21,500 additional troops were a temporary measure.

To that end, five additional American brigades will be deployed to Baghdad over the coming months, tasked with working alongside and embedding with Iraqi security forces. Those troops, roughly 17,500, would double the number of U.S. troops in the city. Another 4,000 troops will be added in Anbar province, Bush said.

“Our troops will have a well-defined mission: to help Iraqis clear and secure neighborhoods, 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,” Bush said.

While “clear, hold and build” is not a new strategy, Bush promised important changes.

“This time, we will have the force levels we need to hold the areas that have been cleared,” he said. “In earlier operations, political and sectarian interference prevented Iraq and American forces from going into neighborhoods that are home to those fueling the sectarian violence. This time, Iraqi and American forces will have a green light to enter these neighborhoods, and Prime Minister [Nouri al-] Maliki has pledged that political or sectarian interference will not be tolerated.”

Other aspects of the plan include increasing the number of American advisers embedded in Iraqi army units; “accelerating” the training of Iraqi forces; doubling the number of Provincial Reconstruction Teams in Iraq; and giving greater leeway for military commanders to spend economic assistance funds.

Democrats immediately attacked the plan.

Sen. John Kerry, the Democratic Party presidential nominee in 2004, said Bush’s new plan should face a congressional vote.

“This plan is neither new nor forward-looking. This is more of what’s taken us backwards.

“There’s no military solution in Iraq,” Kerry said. “There is only a political solution, and the president has no plan to achieve it.

“We’re caught in a civil war in Iraq. Escalation is not the answer. The best answer is to set a deadline to bring our heroes home, force Iraqis to stand up for Iraq, and get Iraq’s neighbors to start taking more responsibility for Iraq’s security.”

Democrats in the new House leadership said they would attempt to block funding for the additional troops.

“We are not going to give the president a blank check. We will subject any proposal to escalate the war to harsh scrutiny, and we will set benchmarks he has to attain to get that money,” Brendan Daly, a spokesman for Speaker of the House Nancy Pelosi, told reporters.

But in his speech, Bush defended the troop increase as necessary and specifically rejected a “phased withdrawal” of American troops.

“To step back now would force a collapse of the Iraqi government, tear the country apart and result in mass killings on an unimaginable scale,” Bush said. “Such a scenario would result in our troops being forced to stay in Iraq even longer and confront an enemy that is even more lethal. 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.”