U.S. President Barack Obama speaks about the war in Afghanistan at the U.S. Military Academy at West Point, N.Y., on Tuesday. (Charles Dharapak/Associated Press)

U.S. President Barack Obama sent his top guns to Capitol Hill Wednesday to sell his new strategy for the war in Afghanistan — but they had trouble convincing congressional committee members the plan will work.

On Tuesday, Obama laid out his plan to send 30,000 additional U.S. soldiers to Afghanistan, and to begin pulling out troops in 18 months, starting July 2011. It was that pullout date that had lawmakers, including Arizona Senator John McCain, asking tough questions.

"Will we withdraw our forces based on conditions on the ground or based on an arbitrary date?" McCain asked. "It's got to be the appropriate conditions or it's got to be an arbitrary date — you can't have both."

Defence Secretary Robert Gates conceded the withdrawal date wasn't quite fixed in stone.

"We will have a thorough review in December 2010. If it appears the strategy's not working and that we are not going to be able to transition in 2011 then we will take a hard look at the strategy itself."

But McCain was not placated: "Then it makes no sense for him [Obama] to have announced the date."

Asked later why any withdrawal date had been set, Gates admitted that American public opinion, which has soured on the war, played a role. Yet in an interview Wednesday morning, Vice-President Joe Biden flatly denied political calculations had anything to do with it.

"The purpose is to make it clear to [Afghan President Hamid] Karzai and his government — who up til now has been unwilling to step up to the ball [and] look to us for everything — that fellas, you got to step up to the ball."

However, that stern message was muddied when Secretary of State Hillary Clinton insisted to senators that Karzai wasn't all that bad.

"He pledged to combat corruption, improve governance and deliver for the people of his country. His words were long in coming but they were welcome."

In Kandahar, Gen. Stanley McChrystal, the top U.S. commander in Afghanistan who had asked for the additional troops, was pleased.

"I think we've got a foundation on which to build where we go forward so I have exceptional confidence right now."

With files from The Associated Press