"Because after all the sacrifices we've made, we want to preserve the gains that you have helped to win and we're going to make sure that Afghanistan can never again, ever, be used again to launch an attack against our country,'' Obama said during remarks to hundreds of U.S. troops at Bagram Air Field, the main American base in Afghanistan.
The president is expected to fill in details of his post-2014 Afghanistan plan during a commencement address Wednesday at the U.S. Military Academy at West Point.
The Pentagon has been pressing for Obama to keep up to 10,000 troops in Afghanistan, though the White House also has been evaluating options that call for fewer forces.
Ben Rhodes, Obama's deputy national security adviser, said the Afghan drawdown, which follows the conclusion of the Iraq war in 2011, marks a "turning point'' for Obama's foreign policy agenda.
"Our foreign policy is going to look a lot different going forward than it did in the last decade when Iraq and Afghanistan really dominated the discussion,'' Rhodes told reporters traveling with Obama to Afghanistan.
Yet it remains unclear exactly how a revamped foreign policy will take shape.
Officials say Obama will continue to take a proactive approach to light-footprint counterterrorism operations the U.S. can undertake on its own, including drone strikes in places like Pakistan and Yemen. But he's also expected to emphasize his desire to have some measure of international consensus when large-scale military options are on the table.
Obama's critics argue that his approach is too cautious and leaves the U.S. beholden to allies who are sometimes less willing to engage.
The debate over launching a military strike on Syria last year shifted in part because British Parliament voted down the use of force, leaving the U.S. with few partners backing an attack. Rather than press forward, Obama decided to seek congressional approval, then signed on to a Russian effort to strip Syria of its chemical weapons stockpiles.
The president's opponents contend that Obama's Syria indecision not only emboldened Russia as it annexed Crimea and eyes more Ukrainian territory, but also China in its land and sea disputes with numerous Asian nations.
White House officials reject the notion that Obama can only show strength by launching military action and argue that economic sanctions are proving to be a deterrent to Russia. They also cast the chemical weapons agreement with Syrian as a sweeping success, even if it has done little to end a civil war there that has left more than 150,000 people dead.
White House officials say Obama will also address the U.S. response to those issues during his West Point speech, as well as during an upcoming trip to Europe.
"What we want to do is step back and put all of these different events into the context of how does America lead in the world and how do we strike that balance between not getting overextended as we were in Iraq, but ensuring that we are leading coalitions of nations, leading the international community on different issues,'' Rhodes said.
The U.S. is counting on a smaller coalition of international forces to stay behind with U.S. troops in Afghanistan after the end of this year. That force presence remains contingent on the Afghan government signing a bilateral security agreement with the U.S.
Despite negotiating the agreement, Afghan President Hamid Karzai has refused to sign the agreement, choosing instead to leave that to his successor. Both of the candidates on the ballot in next month's Afghan presidential runoff have said they plan to sign the accord.
Obama pointedly did not meet with Karzai while in Afghanistan and it appeared as though the U.S. gave the Afghan leader little notice before Obama arrived at Bagram. The White House said the two leaders did speak by phone, but only after Air Force One had departed for Washington.