Speaking from the White House, Obama said he would draw down troops to 8,400 by the end of his administration, a change from the initial target of 5,500. Currently there are 9,800 troops supporting the Afghanistan government in its fight against the Taliban, attempts by al Qaeda to regroup and a nascent threat from ISIS.
"The security situation in Afghanistan remains precarious," Obama said, with Secretary of Defense Ash Carter and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford at his side, a day before leaving for the NATO summit in Poland where he will meet allies also engaged in the Afghan operation.
"I strongly believe it is in our national security interest ... that we give our Afghan partners the best opportunities to succeed," Obama said.
The decision means that Obama, who came to power vowing to concentrate on winning the war in Afghanistan, after what he saw as a diversion by the Bush administration into Iraq, will hand responsibility for America's longest war to his successor. And by adjusting his target for troop numbers, Obama was implicitly admitting -- despite insisting that U.S. forces had forged great progress in Afghanistan -- that the situation at the end of his administration was not as positive as he might have hoped.
"We have to deal with the realities of the world as it is. We can't forget what's at stake in Afghanistan. This is where al Qaeda is trying to regroup, this is where ISIL continues to try to expand its presence," Obama said, using another name for ISIS.
The tone of that comment contrasted with the President's repeated vows to end the Afghan war, including a declaration in December 2014 at the end of U.S. combat operations that "the longest war in American history is coming to a responsible conclusion."
He noted Wednesday that although American forces were no longer engaged in a major land war in Afghanistan, Americans were still continuing to die -- 38 members of the military and U.S. civilians have perished in the country in the last year-and-a-half.
In his remarks on Wednesday, the President also sought to make a distinction between the training and support mission that U.S. troops are now engaged in and the mission that he escalated when he took over from President George W. Bush in 2009 in an apparent attempt to square the political circle of his promises to end costly U.S wars abroad but to leave thousands of American troops behind.
"Compared to the 100,000 troops we once had there, today, fewer than 10,000 remain. And compared to their previous mission, helping to lead the fight, our forces are now focused on two narrow missions: training and advising Afghan forces and supporting counterterrorist operations against the remnants of al Qaeda as well as other terrorist groups, including ISIL," Obama said.
"In short, even as we've maintained a relentless, you know, case against those who are threatening us, we are no longer engaged in a major ground war in Afghanistan."
But Republicans criticized Obama's comments.
South Carolina Sen. Lindsey Graham said it was a more of a political decision than a military one.
"I've never heard anyone suggest that our current level of 9,800 troops in Afghanistan is too high or that President Obama's new 8,400 troop level will be just right," Graham said in a statement.
"But let's be clear -- this troop reduction, while it will seem small to many, will have a negative impact on the security situation in Afghanistan."
Tennessee Sen. Bob Corker, chairman of the Senate Foreign Relations Committee, said he thought the troop levels should remain the same, but praised the timing of Obama's announcement.
"Announcing this decision prior to the NATO summit helps set expectations about our commitment to a stable and democratic Afghanistan and continued support of our allies in this effort," said Corker.
Obama did not mention Iraq during his remarks, but many observers will see his decision as heavily influenced by events that unfolded in the Middle Eastern nation since his decision to bring all U.S. troops home. The rise of ISIS and a sectarian meltdown in Iraq ever since has forced Obama to send hundreds of U.S. troops back to the country to support Iraqi forces.
One difference between the two countries, however, is that Washington considers it has a more effective partner in the Afghan government of President Ashraf Ghani, than it ever had in the chaotic administration of former Iraq premier Nuri al-Maliki.
Obama's announcement on Wednesday was also notable because it almost certainly represented the last of a string of troop review exercises and announcements on troop numbers that have punctuated his administration, focusing on Iraq and Afghanistan, that have prompted Republicans to accuse him of imposing artificial, politically motivated timelines on military operations.
And far from predicting a swift de-escalation of U.S. troop deployments abroad, Obama, who once told Americans that "the tide of war is receding," appeared to lay the ground for prolonged U.S. involvement.
"In January, the next U.S. president will assume the most solemn responsibility of the commander in chief, security of the United States and the safety of the American people. The decision I'm making today ensures that my successor has a solid foundation for continued progress in Afghanistan as well as the flexibility to address the threat of terrorism as it evolves," Obama said.
"Afghanistan is not a perfect place. It remains one of the poorest countries in the world. It is going to continue to take time for them to build up military capacity that we sometimes take for granted," Obama said.
"And given the enormous challenges they face, the Afghan people will need the partnership of the world, led by the United States, for many years to come."