The agreement was long sought by the US-backed government of Afghan President Hamid Karzai, the perpetually skittish leader who has publicly voiced fears of what would befall his country if the United States quickly packed up and left.

"I recognize that many Americans are tired of war," Obama said in the speech. "But we must finish the job we started in Afghanistan and end this war responsibly."

The larger rationale of the agreement was to reassure Afghan leaders that the United States would not repeat the mistake it made in the 1980s. Then, Washington withdrew support for anti-Soviet militia forces in Afghanistan and set the stage for Taliban rule. The Taliban then allowed al-Qaida to use the country to plan the terror attacks of Sept 11, 2001.

In his speech, Obama turned the signing of the promise to stay in Afghanistan into a vehicle for his other promise — to go.

The signing was a quick and businesslike affair at Karzai's palace in Kabul. There were pleasantries, but no pageantry. There was also no opportunity for Karzai to make one of the off-message demands or denunciations of US behavior that have exasperated US officials in the past, even when they acknowledged Karzai had a point.

With that, it was back to the sprawling US air base outside the capital to underscore that last point, that he will close down the war and bring US forces home.

By alighting in Afghanistan on the anniversary of the raid that killed Sept 11 mastermind Osama bin Laden, Obama was also making an unsubtle show of the power of the presidency. Not only is he the commander in chief who can finally end what many Americans see as an unwinnable war — Obama was telling Americans that he is the commander in chief who bagged the biggest bad guy in America's recent history.

Republicans warily saluted Obama's war-zone trip but accused him of craven politics nonetheless.

"Clearly this trip is campaign-related," said Sen Jim Inhofe, R-Okla., a senior member of the Senate Armed Services Committee. "This trip to Afghanistan is an attempt to shore up his national security credentials, because he has spent the past three years gutting our military," a reference to tightening defense budgets.

Obama's presumed Republican opponent, Mitt Romney, was in New York accusing the president of politicizing the fleeting unity that came with bin Laden's death.

Stephen Biddle, a defense analyst at the Council on Foreign Relations, said Obama will be hard pressed to convince Afghans or Pakistanis that the United States will remain an effective security partner once most US troops have gone home.

"The trouble is, he is talking to audiences that have a very strong belief that the United States is going to abandon them," Biddle said in a phone interview.