A total of 10,000 troops will leave the war zone by the end of this year — fulfilling Obama's promise — and more than 20,000 additional forces will leave by the summer of 2012, shortly before the president will go before voters in search of a second term.

Still, almost 70,000 U.S. troops will remain in an unstable country, fighting in a war bound to see more Americans killed. Obama said they will leave at a steady pace, but the U.S. combat mission is not expected to end until December 2014 — and even then, a sizable and enduring contingent may remain in a different role.

Obama's announcement from the White House came in a perilous political environment, with Americans soured on the war and the economy, many members of Congress pushing him to get troops home even faster, and his Republican presidential rivals taking shots at his leadership at every chance. Conceding the economic strain of waging war at a time of rising debt and fiscal constraint, Obama said it was time for America "to focus on nation building here at home."

The withdrawal is supported by the bold bottom-line claims of his security team: Afghanistan, training ground for the Sept. 11, 2001, attacks on America, is no longer a launching pad for exporting terrorism and hasn't been for years. Yet the White House insists the U.S. must maintain a strong fighting force in Afghanistan for now to keep the country from slipping back into a haven for al-Qaida terrorists.