WASHINGTON (CNN) -- President Bush said Thursday night conditions on the ground in Iraq have improved sufficiently to start bringing some U.S. troops home, and urged Americans divided over the war to "come together."

President Bush tells the nation Thursday night that 5,700 U.S. soldiers in Iraq will be sent home by Christmas.

In a televised speech to the nation, Bush said he would reduce U.S. force strength by 5,700 troops by Christmas and, by next July, reduce the number of combat brigades from 20 to 15 -- a decrease of roughly 21,500 troops overall.

The first step in that process will come later this month, when 2,200 Marines leaving Anbar province will not be replaced, the president said.

"The principle guiding my decisions on troop levels in Iraq is 'return on success' -- the more successful we are, the more American troops can return home," Bush said during a 17-minute prime-time address from the Oval Office.

"In all we do, I will ensure that our commanders on the ground have the troops and flexibility they need to defeat the enemy." Watch how Bush bought time with the speech »

In the Democratic response, Sen. Jack Reed of Rhode Island said Bush's speech "does not amount to real change."

"We intend to exercise our constitutional duties and profoundly change our military involvement in Iraq," he said.

The top U.S. commander in Iraq, Gen. David Petraeus, and U.S. Ambassador to Iraq Ryan Crocker told Congress this week that progress had been made on the ground in Iraq since Bush in January sent 30,000 additional troops. The move was termed "the surge."

Petraeus said the surge campaign has met its military goals of reducing sectarian killings by more than 50 percent nationwide and by more than 80 percent in Baghdad.

Bush said Petraeus and Crocker "made clear that our challenge in Iraq is formidable, yet they concluded that conditions in Iraq are improving, that we are seizing the initiative from the enemy and that the troop surge is working."

The president pointed to Anbar province, where security improved after local tribal leaders joined U.S. forces in fighting al Qaeda terrorists.

"The changes in Anbar show all Iraqis what becomes possible when extremists are driven out," Bush said.

"They show al Qaeda that it cannot count on popular support, even in a province its leaders once declared their home base, and they show the world that ordinary people in the Middle East want the same things for their children that we want for ours."

Despite progress in Iraq, Petraeus told Congress this week that it would be premature to announce withdrawal plans for the 130,000 U.S. troops who will remain after the five brigades leave by next summer.

However, he said further reductions might be revisited in his next report in March.

Bush told the American people that in their March report, Petraeus and Crocker "will provide a fresh assessment of the situation in Iraq and of the troop levels and resources we need to meet our national security objectives."

However, he cautioned the process of building a free Iraq "will be difficult," noting that U.S. political, economic and "security engagement" will extend beyond the end of his presidency in 2009.

"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," he said.

Though he hailed security improvements, Bush expressed dissatisfaction at the pace of political change, calling on the Iraqi government to show "the same determination to achieving reconciliation."

"This is an enormous undertaking after more than three decades of tyranny and division," Bush said. "The government has not met its own legislative benchmarks, and, in my meetings with Iraqi leaders, I have made it clear that they must."

He also called on the Iraqi people to "demand that your leaders make the tough choices needed to achieve reconciliation."

"As you do, have confidence that America does not abandon our friends, and we will not abandon you," he said.

The president also turned his attention to Congress and the American people, expressing hope that bitter internal divisions of the war can start to heal.

"Americans want our country to be safe and our troops to begin coming home from Iraq. Yet, those of us who believe success in Iraq is essential to our security and those who believe we should bring our troops home have been at odds," Bush said.

"Now, because of the measure of success we are seeing in Iraq, we can begin seeing troops come home. The way forward I have described tonight makes it possible, for the first time in years, for people who have been on opposite sides of this difficult debate to come together."

The president said that "whatever political party you belong to, whatever your position on Iraq, we should be able to agree that America has a vital interest in preventing chaos and providing hope in the Middle East."

Bush also said critics who charge that "the gains we are making in Iraq come too late" are mistaken.

"It is never too late to deal a blow to al Qaeda, it is never too late to advance freedom and it is never too late to support our troops in a fight they can win," he said.

Bush ended his speech by calling attention to the death of Army Spc. Brandon Stout of Michigan, whose family, the president said, had e-mailed him.

Brandon volunteered for the National Guard and was killed while serving in Baghdad, the president said.

"His family has suffered greatly. Yet in their sorrow, they see larger purpose. His wife, Audrey, says that Brandon felt called to serve and knew what he was fighting for," Bush said.

Stout's parents, Tracy and Jeff, wrote Bush.

" 'We believe this is a war of good and evil and we must win ... even if it cost the life of our own son. Freedom is not free,' " Bush said. E-mail to a friend

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