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On the campaign trail, Sen. Barack Obama offered a pledge that electrified and motivated his liberal base, vowing to “end the war” in Iraq.

But as he moves closer to the White House, the president-elect 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,” Obama said this week as he introduced his national security team.

Publicly at least, Obama has not set a firm number for that “residual force,” though one of his national security advisers, Richard Danzig, said during the campaign that it could amount to between 30,000 and 55,000 troops. Nor has Obama laid out any timetable beyond 16 months for troop drawdowns.

In the meantime, military planners are drawing up tentative schedules aimed at meeting both 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 security agreement between the United States and the Iraqi government.

That agreement remains subject to change, by mutual agreement, and Army planners acknowledge privately that they are examining projections that could see the number of American troops in Iraq hovering between 30,000 and 50,000 for a substantial time even beyond 2011.

In Iraq today, there are 15 brigades defined as combat forces, with one on its way home. But the overall number of troops is more than 50 brigade equivalents, for a total of 146,000 troops.

At a news conference in Chicago this week, Obama emphasized his willingness to listen to senior officers and his new national security team, which includes Defense Secretary Robert Gates.

On Tuesday, Gates 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?” 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.”