 -- President Barack Obama on Monday acknowledged "risks" in his decision to withdraw U.S. combat forces from Afghanistan by the end of 2014 but said war-weary Americans can't wait for that strife-torn country to be "perfect."

"I don't think that there is ever going to be an optimal point where we say, this is all done, this is perfect, this is just the way we wanted it and now we can wrap up all our equipment and go home," Obama said at a press conference at the end of a two-day NATO summit in his hometown of Chicago.

Obama also said the Pentagon isn't giving him an unrealistically rosy picture of Afghanistan. "I can't afford a white-wash. I can't afford not getting the very best information in order to make good decisions."

"I should add, by the way, that the danger a lot of times is not that anybody is purposely trying to downplay challenges in Afghanistan. A lot of times it's just the military culture is 'we can get it done.' And so, their thinking is, 'how are we going to solve this problem?' not boy, 'why is this such a disaster?'"

The alliance, which had already embraced the 2014 withdrawal date, voted to hand Afghan forces the lead in combat operations starting in mid-2013. Building up the country's military and police — and its economy — are vital to the NATO-led International Security Assistance Force's planned draw-down.

"We leave Chicago with a clear roadmap," the president said. "Our coalition is committed to this plan to bring our war in Afghanistan to a responsible end."

"This is a process and it's sometimes a messy process, just as it was in Iraq. But think about it. We've been there now 10 years," Obama said. "And the Afghan Security Forces themselves will not ever be prepared if they don't start taking that responsibility." Obama said the vast NATO-led presence "can be counterproductive," because "no matter how much good we're doing" a decade-long war can put "a strain" on Americans and Afghans alike.

"So I think that the timetable that we've established is a sound one, it is a responsible one," he said. " Are there risks involved in it? Absolutely. Can I anticipate that over the next two years there are going to be some bad moments along with some good ones? Absolutely."

Republicans have criticized the withdrawal plan, saying it will encourage Islamist fighters to wait out the allies and prompt even friendly Afghans to hedge their bets for a day when NATO troops are gone.

On Sunday, the top commander in Afghanistan, General John Allen, stressed that American soldiers would still be in combat — just not in the lead — until they leave at the end of 2014. And Allen said he would have a contingency force ready to help out Afghan troops next year if the Taliban prove too much for them.

The United States is on track to reduce its presence to 68,000 troops by late September. More than 3,000 Americans have been killed in the decade-long conflict launched to catch or kill Osama bin Laden, whom Navy SEALS shot dead in a dramatic May 2011 raid inside Pakistan.

But NATO forces "will continue to train, advise and assist, and support Afghan forces as they grow stronger," Obama said.

"I think it is the appropriate strategy whereby we can achieve a stable Afghanistan that won't be perfect, we can pull back our troops in a responsible way and we can start rebuilding America and making some of the massive investments we've been making in Afghanistan here back home, putting people back to work, retraining workers, rebuilding our schools, investing in science and technology, developing our business climate," he said.

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