WASHINGTON — President Bush has accepted the recommendation of his senior civilian and military advisers to reduce the number of American troops in Iraq by 8,000 in the early months of next year.

The reduction will begin with a Marine Corps battalion set to leave this fall from Anbar Province, once the center of the antigovernment insurgency.

In an address on Tuesday to the National Defense University here, Mr. Bush will unveil his decision on future force levels in Iraq, which includes withdrawing a full brigade of combat troops in the first few weeks of 2009, according to a draft of the speech released late Monday by the White House.

Neither the Marine battalion nor the Army brigade will be replaced, leaving the American combat force in Iraq at 14 brigades. After other support and logistics units are withdrawn under the new orders, the American troop levels in Iraq would drop to about 138,000 by March, still several thousand more than were there in January 2007, when Mr. Bush announced the “surge” that brought the total over 160,000.

“Here is the bottom line: While the enemy in Iraq is still dangerous, we have seized the offensive, and Iraqi forces are becoming increasingly capable of leading and winning the fight,” Mr. Bush says in the speech. “As a result, we have been able to carry out a policy of ‘return on success’ — reducing American combat forces in Iraq as conditions on the ground continue to improve.”

Mr. Bush accepted a consensus set of recommendations presented last week from Gen. David H. Petraeus, the senior Iraq commander; Lt. Gen. Martin E. Dempsey, acting commander of the military’s Central Command; Adm. Mike Mullen, chairman of the Joint Chiefs of Staff; and Defense Secretary Robert M. Gates, according to Pentagon and White House officials.

Mr. Bush will also announce a decision to increase American force levels in Afghanistan by about 4,500 troops, according to the draft of the speech.

“The president’s decision paves the way for us to get even more troops out of Iraq this year and into Afghanistan,” said Geoff Morrell, the Pentagon press secretary. “So the progress our forces are making in Iraq continues to pay big dividends for the commanders in Afghanistan."

A Marine battalion that was scheduled for service in Iraq will instead enter Afghanistan by November. And in January, an Army combat brigade that had been scheduled for service in Iraq will deploy instead to Afghanistan.

The president’s speech also highlights decisions to vastly increase the size of the Afghan National Army, which will grow from its current size of 60,000 troops to 120,000, beyond the 80,000 goal of previous plans. If the progress in Iraq continues, Mr. Bush’s speech says, additional reductions would be possible in the first half of 2009.

Mr. Bush will also highlight the order reducing to 12 months the current 15-month combat tours for Army forces in Iraq, which he will say will “ease the burden on our forces, and make life easier for the military families that support them.”