WASHINGTON - On the campaign trail, Senator Barack Obama offered a pledge that electrified and motivated his liberal base, vowing to "end the war" in Iraq.

Defense Secretary Robert M. Gates, who was consulted several times by President-elect Barack Obama since the election. (Brendan Smialowski/Bloomberg News) But as he moves closer to the White House, President-elect Obama is making clearer than ever that tens of thousands of American troops will be left behind in Iraq, even if he can make good on his campaign promise to pull all combat forces out within 16 months.

"I said that I would remove our combat troops from Iraq in 16 months, with the understanding that it might be necessary - likely to be necessary - to maintain a residual force to provide potential training, logistical support, to protect our civilians in Iraq," Mr. Obama said this week as he introduced his national security team.

Publicly at least, Mr. Obama has not set a firm number for that "residual force," a phrase certain to become central to the debate on the way ahead in Iraq, though one of his national security advisers, Richard Danzig, said during the campaign that it could amount to 30,000 to 55,000 troops. Nor has Mr. Obama laid out any timetable beyond 16 months for troop drawdowns, or suggested when he believes a time might come for a declaration that the war is over.

In the meantime, military planners are drawing up tentative schedules aimed at meeting both Mr. Obama's goal for withdrawing combat troops, with a target of May 2010, and the Dec. 31, 2011, date for sending the rest of American troops home that is spelled out in the new agreement between the United States and the Iraqi government.

That status-of-forces agreement remains subject to change, by mutual agreement, and Army planners acknowledge privately that they are examining projections that could see the number of Americans hovering between 30,000 and 50,000 - and some say as high as 70,000 - for a substantial time even beyond 2011.

As American combat forces decline in numbers and more provinces are turned over to Iraqi control, these military planners say, Iraqi security forces will remain reliant on significant numbers of Americans for training, supplies, logistics, intelligence and transportation for a long time to come.

There always was a tension, if not a bit of a contradiction, in the two parts of Mr. Obama's campaign platform to "end the war" by withdrawing all combat troops by May 2010. To be sure, Mr. Obama was careful to say that the drawdowns he was promising included only combat troops. But supporters who keyed on the language of ending the war might be forgiven if they thought that would mean bringing home all of the troops.

Pentagon planners say that it is possible that Mr. Obama's goal could be accomplished at least in part by relabeling some units, so that those currently counted as combat troops could be "re-missioned," their efforts redefined as training and support for the Iraqis.

In Iraq today, there are 15 brigades defined as combat forces in this debate, with one on its way home. But the overall number of troops on the ground is more than 50 brigade equivalents, for a total of 146,000 troops, including service and support personnel. Even now, after the departure of the five "surge" brigades that President Bush sent to Iraq in January 2006, the overall number of troops in Iraq remains higher than when Mr. Bush ordered the troop increase, owing to the number of support and service personnel remaining.

At his news conference in Chicago on Monday, Mr. Obama emphasized his willingness to listen to the advice from senior officers and that of his new national security team, which includes Defense Secretary Robert M. Gates, the first Pentagon chief in history to continue serving under a newly elected president; Adm. Mike Mullen, chairman of the Joint Chiefs of Staff; and, as national security adviser, Gen. James L. Jones, the retired four-star Marine officer who served as NATO's supreme commander.

Since the election, Mr. Obama has held unannounced consultations with both Mr. Gates and Admiral Mullen, described by Obama aides and Pentagon officials as having focused less on tactics and operations and more on broad, strategic views for American national security. On Wednesday, he made a telephone call to Nuri Kamal al-Maliki, Iraq's prime minister, according to the Obama transition office.

To date, there has been no significant criticism from the antiwar left of the Democratic Party of the prospect that Mr. Obama will keep tens of thousands of troops in Iraq for at least several years to come.

At the Pentagon and the military headquarters in Iraq, the response to the statements this week from Mr. Obama and his national security team has been akin to the senior officer corps' letting out its collective breath; the words sounded to them like the new president would take a measured approach on the question of troop levels.

"I believe that 16 months is the right time frame, but, as I've said consistently, I will listen to the recommendations of my commanders," Mr. Obama said at that news conference on Monday. "And my No. 1 priority is making sure that our troops remain safe in this transition phase, and that the Iraqi people are well served by a government that is taking on increased responsibility for its own security."

An apparent evolution of Mr. Obama's thinking can be heard in contrast to comments he made in July, when he called a news conference to lay out his Iraq policy in unambiguous terms.

"I intend to end this war," he said then. "My first day in office I will bring the Joint Chiefs of Staff in, and I will give them a new mission, and that is to end this war - responsibly, deliberately, but decisively." And in a news conference that month in Amman, Jordan, Mr. Obama acknowledged that the American troop increase had bolstered Iraqi security but declared that he would not hesitate to overrule American commanders and redirect troops in Afghanistan.

Mr. Gates, speaking at the Pentagon on Tuesday, a day after he appeared with Mr. Obama to announce the new national security team, made clear that the direction of troop levels now had been decided, with the only decisions remaining on how fast and how low.

"And so the question is, How do we do this in a responsible way?" Mr. Gates said. "And nobody wants to put at risk the gains that have been achieved, with so much sacrifice, on the part of our soldiers and the Iraqis, at this point."