Washington ? As military units move in and out of Iraq over the next six months, the planned rotations provide commanders with opportunities to reduce troop levels. One chance could come as early as January.

Gen. David Petraeus, the top commander in Iraq, has yet to lay out his recommendations for a future war strategy. But improving security conditions in western Anbar province could allow a brigade or Marine unit there to head home after the first of the year without being replaced.

As an example, the 1st Brigade, 3rd Infantry Division, which is currently in Anbar, would hit its 12-month deployment mark in January and could return to Fort Stewart, Ga., early in 2008 if security conditions allowed. Not replacing the brigade would immediately lower troop levels.

Military planners have been poring over deployment schedules and monitoring trends of attacks and violence in Iraq in an effort to map out what commanders call their battlefield geometry.

They want to move troops out of Iraq without sacrificing security gains they have made and without forcing the military repeatedly to shift units around to cover battleground left open by departing troops.

There is broad consensus that Petraeus can start reducing troop levels from 20 brigades to 15 beginning early in the spring by not replacing the five combat brigades that President Bush ordered to Baghdad this year for the buildup.

The first of those five brigades will reach the end of its 15-month tour around April. So if the five were to leave on time over the subsequent months, then by the end of the summer troop levels would be back to what was considered the baseline last year: 15 combat brigades, or about 135,000 forces.

At least five of the brigades now in Iraq are scheduled to leave in the next few months. But all are being replaced by other units, including three brigades from the 101st Airborne Division, from Fort Campbell, Ky.