WASHINGTON – President Bush said Thursday night that improved security in Iraq would allow him to begin bringing troops home, endorsing a proposed pullout that drew skepticism earlier in the week from congressional critics in both parties.

Adopting the proposal laid out by Gen. David H. Petraeus, the top U.S. commander in Iraq, Bush plans to withdraw at least 21,500 troops by next summer, a number that could leave more American military personnel in the war zone than when he began a buildup in January.

Bush spoke near the end of a week of intense Washington focus on the war; it began with testimony by Petraeus and Ryan C. Crocker, the U.S. ambassador to Iraq, and will end Friday with the delivery to Congress of the administration’s latest report on the results of the troop buildup and the broader political conditions in Iraq.

The report, ordered by Congress, is expected to present a pessimistic assessment of the Iraqi government’s ability to make progress toward broad reconciliation.

In his televised address, the president emphasized his often-stated belief that American troops must remain in Iraq until it is able to govern and defend itself.

“The success of a free Iraq is critical to the security of the United States,” Bush said. “Realizing this vision will be difficult — but it is achievable. Our military commanders believe we can succeed. Our diplomats believe we can succeed. And for the safety of future generations of Americans, we must succeed.”

Bush’s central message was that the buildup was having success – and that this progress meant that it was possible to meet U.S. security goals in Iraq and also withdraw U.S. troops. “The principle guiding my decisions on troop levels in Iraq is “return on success,’ ” he said. “The more successful we are, the more American troops can return home.”

He said that those who see “success” in Iraq as crucial to American security and those who want to bring the troops home “have been at odds.” “Now, because of the measure of success we are seeing in Iraq, we can begin seeing troops come home,” he said.

The key to future troop levels, administration officials insist, will be conditions within Iraq and not timetables set by Congress.

Sen. Jack Reed of Rhode Island, a former Army paratrooper delivering the Democrats’ response , said the United States was “eager for change in Iraq,” and that Bush “failed to provide either a plan to successfully end the war or a convincing rationale to continue it.”

He said the president had given the troops neither a clear nor achievable mission, and that “an endless and unlimited military presence in Iraq is not an option.”

As he has throughout the war, Bush drew on the symbols of the presidency, in this case the Oval Office and the importance attached to a prime-time televised address, in an effort to sway public opinion.

Bush said his surge of 30,000 troops has made Iraq safer, in both Anbar province, the stronghold of the Sunni insurgency, and in Baghdad, where rival Sunni militants and Shiite militiamen have battled for control of neighborhoods.

“The goal of the surge is to provide that security — and to help prepare Iraqi forces to maintain it,” he said. “Our success in meeting these objectives now allows us to begin bringing some of our troops home.”

Bush acknowledged that, despite the military gains detailed by Petraeus, the Iraqi government has done little to bring ethnic and religious factions together. Bush’s initial argument for the surge was to give the Iraqi government breathing room to craft critical legislation to help reunite the nation.

With that security delivered as a result of the increased U.S. presence, Bush said, “Now the Iraqi government must bring the same determination to achieving reconciliation.”

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Even as the administration points to developments it says show that Iraq is becoming safer, daily reports from Iraq present evidence of the difficulties it faces, and continuing deaths of U.S. troops — approaching 3,800 combat fatalities since the war began — demonstrate the cost of the mission.

On Thursday, it was the death, by a bomb that exploded outside his home, of a Sunni sheik, Abdul Sattar Rishawi, and two bodyguards. Bush had met with him and other Iraqi political leaders 10 days earlier in Anbar province, in a conference that was cited as evidence of the sort of political reconciliation that has been so difficult to achieve there.

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Although Bush said he planned to bring troops home, he did not specify how many. Senior administration officials said the number was at least 21,500, short of the 30,000 combat and support troops in the surge. Bush also did not address how many troops he expected to remain in Iraq and for how long.

Citing Petraeus’ recommendations, the president said that the initial drawdown would bring 5,700 troops home by Christmas and that by July the 20 U.S. combat brigades would be reduced to 15.

Mowaffak Rubaie, Iraq’s national security advis e r, said at a news conference Wednesday that if Iraq’s forces are were sufficiently prepared and security threats were down, the number of foreign troops there could fall below 100,000 by the end of 2008. But he emphasized that security circumstances — the threats from within the country and beyond its borders, and the ability of Iraqi forces — would be central to that determination.

When Bush announced the buildup, 132,000 American troops were stationed in Iraq. That number has grown to 169,000, its highest point since the war began, and with overlap during turnover periods it is expected to reach 171,000.

The president said Iraq was “fighting for its survival.”

Using much the same argument he has since the first year of the war — that terrorists fighting in Iraq are, ultimately, seeking to attack the United States — Bush said: “Terrorists and extremists who are at war with us around the world are seeking to topple Iraq’s government, dominate the region and attack us here at home.”

With Iraq having placed its trust in the United States, he added, “our moral and strategic imperatives are one: We must help Iraq defeat those who threaten its future — and also threaten ours.”