* Obama's escalation challenges U.S. military

* Withdrawal to begin before top general had forecast

* NATO will not bring Afghan surge to 40,000 troops

WASHINGTON, Dec 2 - U.S. President Barack Obama's escalation of the war in Afghanistan challenges his generals to do more with slightly less than they wanted -- and much, much faster. The odds are against them.

The deployment of 30,000 more troops is a victory for proponents of the buildup within the U.S. military, who say it will breathe new life into a flagging eight-year-old war that would have ended in defeat otherwise.

But Obama did not give General Stanley McChrystal, the top U.S. and NATO commander on the ground, all of the 40,000 troops he sought -- let alone a more ambitious plan to deploy up to 80,000 extra troops. NATO will not entirely bridge the gap.

Obama, in his speech on Tuesday, also demanded success more quickly, calling for U.S. forces to start pulling out in mid-2011 -- a year and a half sooner than McChrystal thought was advisable.

He is being asked to do more with less, at least less than he had asked for, said military analyst Kimberly Kagan, who was part of the team that helped McChrystal assess the Afghan conflict during the summer.

Is it enough? I don't know ... 30,000 forces is a lot of combat power and it has a chance, and I think a reasonable chance, of dramatically changing the situation on the ground.

Commanders are expected to funnel the forces into Taliban strongholds in southern Afghanistan, including Kandahar and opium-producing areas of Helmand province, protecting key population centers.

The reinforcements, all of which are expected to be in place by the end of August, will aim to isolate the Taliban and create the space needed to train the Afghan security forces and build local governance.

TROOPS CAN'T WIN WAR

McChrystal, a straight-talking former special operations commander said to sleep just four or five hours a night, led Americans into an autumn of soul-searching about the Afghan war after his bleak assessment of the conflict was leaked to the media in September.

He warned the war effort would fail without additional troops, and, worse still, that an escalation alone could not ensure victory.

But his calls to address a crisis of confidence in the Afghan government have raised uncomfortable questions about nation-building -- a grim prospect in an underdeveloped country ravaged by three decades of war, rampant corruption and a booming opium trade.

Obama, in his speech, said Afghanistan's government had to crack down on corruption and deliver services to the people, warning the days of providing a blank check are over.

Defense Secretary Robert Gates has announced plans to tighten control of multimillion-dollar U.S. contracts as a first step to fighting corruption.

After three months of White House strategy reviews, McChrystal commended Obama for giving him a clear military mission and the resources to accomplish our task -- even though he did not get all of the troops he wanted.

McChrystal cited the goal of ramping up training of Afghan security forces -- a crucial step for any handover. There are now about 190,000 Afghan soldiers and police, a number McChrystal has recommended raising to 400,000.

But officials have also spoken bluntly about the obstacles of developing capable Afghan security forces given high attrition rates among recruits and widespread illiteracy.

Less than a third of the Afghan population are literate and officer training includes teaching officers to read, a fact McChrystal warns could take the training well beyond 2013.

IT'S NOT IRAQ

For McChrystal and his boss, General David Petraeus, the Afghanistan mission will be a crucial test of counter-insurgency strategy and the lessons of Iraq.

There, a quick surge of 20,000 U.S. troops sent to Iraq between January and July 2007 is one of the factors credited with helping to pull that country back from brink.

But both McChrystal and Petraeus have been wary of drawing comparisons to Iraq given the major differences with Afghanistan, which lacks a strong central government or tribal structure after three decades of war.

For his part, Obama rejected comparisons to Vietnam in his speech on Tuesday, as U.S. generals have done for years.

And then there is the Soviet Union. U.S. military strategists are working hard to make sure that Western forces are not seen as occupiers, as they escalate past the peak size of Soviet forces before their withdrawal in defeat in 1989.

Gates has said showing Afghans that U.S. troops improve their lives will help prevent comparisons to the Soviets. He wants U.S. forces to build wells, schools and roads after clearing towns of the Taliban.

But avoiding such comparisons could be difficult. Even the strategy itself drew allusions to the Soviet Union on Tuesday.

The McChrystal plan calls for what the Soviets did -- holding the cities, said Christine Fair, a professor at Georgetown University's Center for Peace and Security Studies.

That did not work for the Soviets and I don't know how it will work with us, given the troop footprint we have. (Additional reporting by Sue Pleming; Editing by John O'Callaghan) ((phillip.stewart@thomsonreuters.com; +1 202 354 5827; Reuters Messaging: phillip.stewart.reuters.com@reuters.net))