President Barack Obama's announcement to withdraw half the US force in Afghanistan shows his determination to end the war, but leaves open the question of long-term American support for Kabul.

In his State of the Union address on Tuesday, Obama unveiled plans to scale back American forces from 66,000 to 32,000 within 12 months, as part of a long-standing goal by Washington and its allies to pull out nearly all combat troops by the end of 2014.

"Tonight, I can announce that over the next year, another 34,000 American troops will come home from Afghanistan. This drawdown will continue. And by the end of next year, our war in Afghanistan will be over," Obama said.

The move reflects the administration's commitment to turn the page on the conflict, after concluding that a large-scale counterinsurgency campaign was not worth pursuing amid stalemate on the battlefield and more than a decade of troop casualties, analysts said.

"The administration and the president have been very clear over the last 18 months or more that the drawdown would continue at a steady pace. It just represents a continuation of the administration's plan," said David Barno, a retired general who commanded US troops in Afghanistan from 2003 to 2005.

Although most Americans have grown weary of the war, some Republican lawmakers worry the withdrawal pace is too fast and will play into the hands of Taliban insurgents American troops have been fighting since 2001.

But officials say US and NATO forces need to start handing more responsibility to Afghan government forces and that senior officers on the ground will have flexibility to adjust the drawdown as needed.

"The commanders will have discretion on the pace" of the withdrawal, a senior defense official said. And the "focus will be keeping as many forces in play until after the fighting season" ends in the fall, said the official, who spoke on condition of anonymity.

For US troops, the "primary mission is to support the Afghan forces to make them effective in the fighting against the Taliban, not to do the job for them," said Barno, a fellow at the Center for a New American Security, which has a close relationship to the Obama administration.

While Barno believes Afghan forces are "far more capable" than Western experts give them credit for, some in and outside the Pentagon say hard-won progress against the Taliban could unravel because of Obama's troop withdrawal schedule.

"It's a big number of troops to withdraw in a short amount of time," said Thomas Donnelly of the right-leaning American Enterprise Institute.

Instead of waging war against the Taliban, the priority inevitably will shift to managing a large-scale withdrawal, he said.

"Just because of the size of the drawdown and the complexity of operations in Afghanistan, a lot of the command is going to be focused on the drawdown."

Obama has yet to say how many troops he wants to keep in Afghanistan after the NATO withdrawal in 2014, but officials have indicated the White House wants a much smaller presence than initially favored by the Pentagon.

Top generals at one point proposed 15,000 to 20,000 boots on the ground, but officials say the White House favors a light footprint of several thousand.

As US diplomats negotiate an agreement with Kabul for a post-2014 mission, the administration has yet to articulate what the American commitment to Afghanistan will be in the future, and how many billions the US government needs to spend to keep the Kabul government afloat.

"Obama needs to lay out a clear strategy for Afghanistan for the months and years ahead," Michael O'Hanlon of The Brookings Institution wrote on the Politico news website.

Historians, recalling the Soviet occupation of Afghanistan, have noted that Moscow's allies remained in power after Russian troops left, but fell once Moscow stopped funding the regime.

With the current war at a stalemate, the crucial question is not how many US troops stay in Afghanistan, but how long the US Congress will be willing to bankroll the Afghan army and police to keep insurgents at bay, said Stephen Biddle, a professor of political science and international affairs at George Washington University.

"The issue is whether the US Congress is going to keep funding the ANSF (Afghan National Security Forces) to keep them in the field and is there any way to end the war satisfactorily before the Congress cuts off the money," he said.