The President will inform Congress and the American people that thousands of US troops will be back home from Afghanistan in one year's time.
That's about half the U.S. forces currently serving there, and marks the next phase in the administration's plans to formally finish the war by the end of 2014. The U.S. now has 66,000 troops in Afghanistan, down from a peak of about 100,000 as recently as 2010.
The U.S. is still finalizing plans for the size and scope of its military presence after the war ends. The White House has said it would be open to leaving no troops in Afghanistan, though it's likely that a small presence will remain, in keeping with the Pentagon's preferences.
Obama won't announce troop numbers beyond 2014 in Tuesday's speech and has not yet made that decision, said the official, who requested anonymity in order to discuss the troop drawdown ahead of the president.
Obama discussed the next phases of the drawdown with Afghan President Hamid Karzai during a day-long meeting in Washington last month, the first meeting between the two leaders since Obama's re-election. The two leaders agreed to accelerate their timetable for putting Afghan forces in the lead combat role nationwide, moving that transition up from the summer to the spring.