WASHINGTON — The U.S.-Iraqi deal on troop withdrawals, while not yet final, appears to mark the beginning of the end of a combat commitment that has cost more than 4,100 U.S. lives and at least $500 billion.

It does not mean the war is over, or even that most U.S. troops will be home soon. But it shows a new U.S. readiness to set at least a rough timetable for reducing its presence over the next three years. And it reflects a growing U.S. willingness to let Iraq take over the fight against insurgents.

It also coincides with the prospect of a deepening U.S. combat involvement in Afghanistan in coming months. American commanders say more troops are needed there to fight a resurgent Taliban movement that was removed from power by U.S.-led forces after the Sept. 11, 2001, terrorist attacks. Adding to the number of American combat troops in Afghanistan depends on reducing the numbers in Iraq.

Until very recently the Bush administration resisted setting any timetables for concluding American combat involvement in Iraq, insisting that troop reductions be dictated only by developments on the ground as assessed by U.S. commanders. In fact, developments have turned more positive in recent months, even as strains on the U.S. military have grown in the sixth year of an unpopular war.

“The stars appear to be aligning” in a way that optimists would say points to a winding down of the war, said Graham Allison, director of Harvard’s Belfer Center for Science and International Affairs.

“There is a convergence of interests now in a change in the roles and missions for American forces and for reductions of (troop) numbers” on at least a theoretical timeline, Allison said in a telephone interview Friday. He added, with emphasis, that it would be unwise to assume there will be no setbacks

Also, as the White House reminded on Friday, there is not yet a final agreement.

President Bush and Iraqi Prime Minister Nouri al-Maliki spoke during the day by secure video as work on the plan to withdraw U.S. troops continued.

“There are still discussions ongoing,” said spokesman Gordon Johndroe, with the president in Texas. “It’s not done until it’s done. And the discussions are really ongoing. And ongoing and ongoing. But hopefully drawing to a conclusion.”

Wars take unforeseen turns, and it remains possible that a new cycle of mass violence in Iraq could be triggered by any number of remaining sectarian tensions or political conflicts. But at this quieter stage of the war the U.S. has turned clearly in the direction of ending its combat involvement.

Iraqi and American officials said Thursday after Secretary of State Condoleezza Rice visited Baghdad that the two sides agree on a plan for scaling back U.S. forces. But some aspects are still being discussed, and an accord remains subject to approval by top Iraqi leaders and the Iraqi parliament.

The withdrawal deal, part of a broader accord on the future U.S.-Iraqi security relationship, would have American troops moving out of Iraqi cities by next summer as a prelude to a broad pullout of combat forces by the end of 2011, according to Iraqi and American officials involved in the talks.

There are now about 146,000 U.S. troops in Iraq; fewer than half are considered combat forces. The rest perform a wide range of support functions like transportation, administration and maintenance.

It is likely, but not certain, that a residual U.S. force would remain beyond 2011 to continue training and advising the Iraqi security forces, which are still lacking in areas of combat support such as intelligence, surveillance, logistics and air power. Thus while the American commitment of combat forces appears to be headed toward an end, the U.S. seems likely to remain involved in other ways.

This outlook is what Bush’s successor will face in January. The Democratic contender for the White House, Sen. Barack Obama, has said he would withdraw all U.S. combat forces within 16 months of taking office; his Republican rival, Sen. John McCain, says withdrawals must be linked to security progress.

The Iraqis have their own reasons for wanting the U.S. troops out. They are planning to hold provincial elections late this year, followed by national balloting in 2009. The presence of American troops is a political irritant for the Iraqis, even as they recognize their dependence on foreign assistance.

It is unclear how long it might take to complete the emerging U.S.-Iraq security deal. It probably will still be in debate in Baghdad when Gen. David Petraeus, the top U.S. commander in Iraq, makes his much-anticipated recommendation to the Pentagon and the White House on when to resume the drawdown of American forces. The reductions were suspended in July to give Petraeus a chance to assess conditions.

His recommendation, expected by the end of this month, is likely to open the door for Bush to announce at least modest additional troop cuts before the November election.

— Robert Burns has covered national security and military affairs for The Associated Press since 1990.