WASHINGTON – Commanders in Iraq have decided to begin the drawdown of U.S. forces in volatile Diyala province, marking a turning point in the U.S. military mission, The Associated Press has learned.

Instead of replacing the 3rd Brigade of the 1st Cavalry Division, which is returning to its home base at Fort Hood, Texas, in December, soldiers from another brigade in Salahuddin province next door will expand into Diyala, thereby broadening its area of responsibility, several officials said Tuesday.

In this way, the number of Army ground combat brigades in Iraq will fall from 20 to 19. This reflects President Bush’s bid to begin reducing the American military force and shifting its role away from fighting the insurgency toward more support functions like training and advising Iraqi security forces.

The December move, which has not yet been announced by the Pentagon, was described to the AP by Col. Stephen Twitty, commander of the 4th Brigade, 1st Cavalry, in a telephone interview Tuesday. It was confirmed by three other officials in Iraq, including Lt. Col. Michael Donnelly, chief spokesman for the commanding general of U.S. forces in northern Iraq, Maj. Gen. Benjamin Mixon.

The idea is to avoid vacating a contested area, like Diyala, which is northeast of Baghdad, while beginning Bush’s announced reduction of at least 21,500 troops, of which 17,000 were sent to the Baghdad area last spring.

The shift in Diyala in December could be a model for follow-on reductions next year, with a redrawing of the U.S. lines of responsibility so that a departing brigade has its battle space consumed by a remaining brigade. At the same time, Iraqi security forces would assume greater responsibility.

Diyala province is a battered landscape of warring tribes, fertile valleys and pockets of al-Qaida fighters. The sectarian and tribal chasms are wide. Commanders cited signs of substantial progress in the months since thousands of U.S. and Iraqi forces stormed the provincial capital of Baqouba in June.

The unit leaving in December, the 3rd Brigade of the 1st Cavalry, has been in Iraq since October 2006. When it leaves, the 4th Stryker Brigade of the 2nd Infantry Division, now in Salahuddin province, will add Diyala to its area of responsibility.

Donnelly said that even though the number of combat brigades in Iraq will drop by one with the departure of the 3rd Brigade of the 1st Cavalry, the total number of soldiers in northern Iraq will remain almost constant. That is because later in December a unit arriving from Fort Hood – the 3rd Armored Cavalry Regiment – has substantially more soldiers than the unit it will replace.

It is not yet clear how the rest of the five-brigade reduction will be carried out; the cuts are to be completed by July 2008, under a plan recommended by Gen. David Petraeus, the top commander in Iraq, and announced by Bush in September. It probably will include some fresh reductions in the western province of Anbar, where insurgent violence has declined substantially this year.

When Bush announced on Sept. 13 that security in Iraq had improved enough to permit the withdrawal of five brigades between December and next July, commanders said they had not yet figured out how it would be done. The reductions will shrink the U.S. force from 20 combat brigades to 15, which is the total that prevailed before Bush decided in January to add five brigades as the centerpiece of a new strategy designed to tamp down sectarian violence mainly in Baghdad.

The 4th Stryker Brigade that will expand its battle space into Diyala province is one of those five so-called surge brigades.