WASHINGTON --For all the fierce debate over withdrawing troops from Iraq, no one has been able to shed light on the main question: How many troops are we talking about?
Virtually everyone, from the White House to the Democratic presidential candidates, agrees that some of the 156,247 men and women in Iraq, as of July 15, eventually must be withdrawn.
"Bringing our troops home," President Bush said last week, "is a goal shared by all Americans."
Pulling out all combat units, as some have demanded, would reduce the force by less than half, leaving more than 80,000 support troops in Iraq without protection and allowing the insurgency to run rampant.
To avoid that, nearly everyone also agrees that some combat forces should remain in Iraq to fight foreign insurgents, to train the Iraqi army and police, and to protect remaining American troops, diplomats and contractors.
How many soldiers and Marines will that take?
"I think in the tens of thousands," said Lee Hamilton, who co-chaired the Iraq Study Group, a bipartisan panel that recommended in December that the U.S. begin scaling back its military operations in Iraq.
The commander of U.S. ground forces in Iraq, Lt. Gen. Ray Odierno, said he hadn't been asked.
"What do they want me to achieve? Once I'm given that, I'll be able to give you an assessment of what's needed," he told a recent Pentagon briefing, adding that it could take until November to make such an assessment.
Yet the question could become more pressing long before then.
While Senate Democrats have set aside, for now, their drive to force Bush to begin a troop withdrawal, that effort will likely intensify in September, when Gen. David Petraeus, the top commander in Iraq, is due to report with U.S. Ambassador Ryan Crocker on progress under the 28,000-man "surge" that the president ordered in January.
At the same time, the Pentagon is running out of fresh troops to maintain the current force of almost 160,000.
In March, many of those troops are scheduled to rotate home after 15 months in combat. The Army says it will have "a very difficult time" finding enough troops to replace them, a staff officer said.
By April, the military's ability to sustain current troop levels in Iraq "vanishes," Reed, a former Army paratrooper, said July 13 on CSPAN.
For these political and military reasons, "it is likely that there will be changes in military missions and force levels as the year proceeds," said Republican Sen. Richard G. Lugar of Indiana.
Already, the 74,600 combat soldiers and Marines in Iraq are far too few to carry out the counter-insurgency strategy developed last fall by Petraeus. That strategy, enshrined in U.S. Army Field Manual FM100-34, calls for 20 troops for every 1,000 persons in the local population, a formula which would put 120,000 soldiers just in Baghdad.
For those advocating even further reductions, matching troop numbers to a specific military mission has been difficult.
For example, a plan championed by Sen. Carl Levin of Michigan, the Democratic chairman of the Armed Services Committee, would require that a much smaller U.S. combat force be authorized to fight only those insurgents associated with al-Qaida, but not other insurgents, a distinction that could be difficult to make in a firefight.
A plan advanced by Illinois Sen. Barrack Obama, a Democratic presidential candidate, would remove "all United States combat brigades" from Iraq - except those needed to fight terrorists, train Iraqis, protect Americans "and other purposes" the president may decide. No numbers were specified.
Republican Sen. John W. Warner of Virginia and Lugar urge that U.S. forces be refocused on these same missions, which would require "some level" of troops, their proposal said.
Outside the military, a lively chorus of civilian analysts and academics has been at work figuring out how small the U.S. combat force should become.
Shrinking the total number is difficult because of the enormous support community needed to maintain combat troops in the field: headquarters staffs, intelligence, medical, communications and logistics specialists, civil and combat engineers, civil affairs and explosive ordnance detachments, technicians, mechanics and others.
At present, roughly 80,000 American troops provide these services, including about 6,000 military personnel working as advisers and trainers with Iraqi police or Army units, and about 3,000 Special Forces soldiers, sailors and airmen. In addition, at least 20,000 American civilians work in Iraq as contractors.
All of them need bases in which to operate, and the bases must be supplied. At present, 2,000 trucks are on Iraq's roads every day in normal supply operations.
That support should be sharply reduced to a leaner force, some analysts said.
"I'm saying take the force down to 100,000 immediately, and then to 50,000 to 60,000," said Frank Hoffman, a retired Marine officer and well-known strategic analyst in Washington. "Of course, that means you have to do without your dentists and chaplains, and go without ice cream every night."
The Center for a New American Strategy, a centrist think-tank in Washington, proposed a gradual reduction to 60,000 troops by 2009, a force that would include a substantial expansion of American advisers to 20,000 soldiers, more than triple the current number.
"That's back-of-the-envelope planning," said James Miller, a senior vice president of the center and former Pentagon war planner. "Our real recommendation is that the military should do this."
But a smaller combat force may itself be problematical.
For a force that is half of today's size, "it's very hard to find something for them to do that is simultaneously safe and useful," said Steven Biddle, senior analyst on defense issues for the Council on Foreign Relations.
"So you end up with a kind of worst-of-both-worlds situation," Biddle said in an e-mail interview. "You have too few troops to do anything useful, but too many to reduce casualties to an acceptable level."