WASHINGTON — President Bush laid out his "new way forward" in Iraq last night, calling for beefing up U.S. forces by extending tours of duty for 21,500 troops, adding $1.2 billion in reconstruction aid and having Iraqi forces take the lead in joint combat operations.

The plan calls for an aggressive campaign against Sunni insurgents and Shiite militias — something that current and former military officials warned will likely touch off a more dangerous phase of the war, including months of fighting in the streets of Baghdad.

That is exactly the sort of urban fight that war planners strove to avoid during the spring 2003 invasion of Iraq. And military officials said sustaining it for more than a few months would further strain U.S. forces already burdened by the war.

"Many listening tonight will ask why this effort will succeed when previous operations to secure Baghdad did not," Bush said on national television last night.

The answer, he said, is that the new effort would have more resources and that Iraqi officials have given the U.S. "a green light" to take action in all neighborhoods of the capital. Powerful members of the Iraqi government have been shielding some areas from security crackdowns, often to protect sectarian supporters.

The renewed campaign in Baghdad will target all insurgents and militias, which means the U.S. military will wind up fighting the Madhi Army of radical cleric Muqtada al-Sadr. That militia is estimated by some U.S. intelligence officials to have grown over the past year to about 60,000 fighters. Some in the Pentagon consider it more militarily effective than the Iraqi army.

Al-Sadr is one of the most powerful figures in the Iraqi government, and he has forced it and the U.S. military to back down in the past. Yet if his army is not confronted, some analysts said, the entire offensive may falter and the sectarian conflict may intensify, because Sunnis will feel it is just one more way of attacking them while leaving Shiite death squads free to operate.

"If our troops do not enter Sadr City, they belittle the notion of a surge because they would leave a leading militia unscathed," said Patrick Cronin of the International Institute for Strategic Studies, a think tank.

'MISTAKES' MADE

Bush admitted in his speech that his current strategy had failed.

"The situation in Iraq is unacceptable to the American people — and it is unacceptable to me," Bush said, acknowledging diminished public support for the war and an increasingly assertive opposition in the new Democratic-controlled Congress. "It is clear that we need to change our strategy in Iraq."

The new plan calls for assigning top priority to ending the sectarian violence in key areas of Baghdad, assigning primary responsibility for security to the Iraqi army and police, and shifting U.S. troops into a supporting role — including embedding more Americans with Iraqi units.

In a striking concession, Bush said that the last year in Iraq turned out to be "the opposite" of what he had expected — an explosion of sectarian violence, instead of growing national unity among the Iraqi people and a winding down of American military involvement.

He said that occurred in part because there had not been enough troops to provide security in Iraqi neighborhoods — a strong criticism of his policy since the earliest days of the invasion.

"Where mistakes have been made, the responsibility rests with me," Bush said. "It is clear that we need to change our strategy in Iraq."

FOCUS ON BAGHDAD

But he argued that the change should be to increase, not decrease, the U.S. commitment to stabilize the troubled country he invaded and occupied more than three years ago. "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."

Until now, the focus of U.S. efforts has been to "stand up" Iraqi security forces and rely on the Iraqi government to promote a political reconciliation to defuse the insurgency and reduce violence.

Now, after months of increasing sectarian violence and ineffective efforts by Prime Minister Nouri al-Maliki to bring Sunnis into the government, Bush acknowledged that the Iraqi government and its security forces cannot control the intensifying conflict between the Sunni minority that long dominated Iraq and the Shiite majority now determined to take control of the country.

As a result, the president said, U.S. troops must effectively combine forces with the Iraqis and take on the mission of quelling violence — especially on the streets of Baghdad.

"Most of Iraq's Sunni and Shia want to live together in peace, and reducing the violence in Baghdad will help make reconciliation possible," Bush said.

$5.6 BILLION COST

He insisted that U.S. troops will act primarily in support of Iraqi forces as they implement a new security plan drawn up by al-Maliki's government.

Bush's words contained an unspecified warning that at some point, the United States' commitment will end: "I have made it clear to the prime minister and Iraq's other leaders that America's commitment is not open-ended," he said.

"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. Now is the time to act. The prime minister understands this."

At the same time, Bush avoided setting the kind of benchmark deadlines for progress that some of his critics have demanded, and he continued to insist that the U.S. cannot afford to fail in Iraq.

The increased deployment will bring an additional 21,500 U.S. troops to Iraq, on top of the 140,000 there now. To pay for the added troops and the economic program, senior administration officials said, Bush is likely to seek an additional $5.6 billion in a supplemental spending request submitted apart from the 2008 budget proposal he is expected to deliver to Congress on Feb. 5.

Although the troop numbers are slightly lower than some advocates of a buildup have advocated, those who have backed a larger force were cautiously optimistic, noting that the number of Army brigades and the Marine regiment being sent are largely in line with their recommendations.

Bush will be sending five Army brigades to Baghdad, and the equivalent of one Marine regiment to Anbar, only slightly less than the five brigades and two Marine regiments recommended by retired Army Gen. Jack Keane, an influential backer of the buildup.

"We were very careful to focus on the number of brigades, and that's what to look for," said Frederick Kagan, an analyst at the American Enterprise Institute who co-wrote an influential strategy for Iraq with Keane that was largely embraced by the White House.

Military officials say the tactical changes that will accompany the troop increase are, in many ways, more important than the additional troops themselves. Army Gen. John Abizaid, the outgoing Middle East commander, has resisted increasing the visibility of U.S. forces in Baghdad, arguing that it would only inflame Iraqi civilians and prevent Iraqi forces from taking on the security burden themselves.

The new plan reverses those tactics, however, and will see troops move out from the handful of large bases Abizaid established into smaller outposts throughout the city.

A senior administration official said battalion-sized Army units — about 400 to 600 troops each — will fan out to each Baghdad district in an effort to hold and rebuild areas that in the past were frequently backsliding into sectarian violence once U.S. troops left.

Such tactics are closely associated with counterinsurgency strategies developed by Army Lt. Gen. David Petraeus, the incoming commander in Iraq, who has spent the past two years developing doctrine on irregular warfare at the Army's leading think tank at Fort Leavenworth, Kansas.

KANE'OHE MARINES

According to a defense official, the troop increase will total 21,500 and will be achieved by sending one new Army brigade to Baghdad — a unit of the 82nd Airborne Division, which is already en route to Kuwait — and speeding up the deployment of other Army units that were to go to Iraq this spring.

Two Marine battalions currently in Iraq that were due to depart next month — including one based at Kane'ohe Bay — will also see their rotation extended to provide the 4,000 additional troops in western Anbar province, according to the defense official.

The White House announced that it will remobilize units of the Army National Guard to help sustain the buildup, a potentially controversial move that will force Guard brigades into second tours in Iraq.

Until now, Pentagon policy has only required Guard units to deploy overseas once every six years.

Bush also recommended increasing the overall size of the Army and Marine Corps.

The Los Angeles Times, Washington Post and McClatchyTribune News Service contributed to this report.