An Army paratrooper unit that led the U.S. troop surge into Baghdad last year is returning home, marking a resumption of a troop withdrawal that will test the durability of recent security gains.

The pullout began last December with the departure of one brigade, numbering about 5,000 troops, taking the overall U.S. troop level in Iraq to 158,000. But a three-month lull was built into the drawdown plan, during which commanders saw insurgent violence shift from Baghdad to northern Iraq.

Although it has not yet been publicly announced, a senior military official said Tuesday that the 2nd brigade, 82nd Airborne Division is heading back to North Carolina in coming days and will not be replaced in the rotation. The official spoke on condition of anonymity because the movements are not completed.

That will drop the number of U.S. combat brigades in Iraq from 19 to 18, with an additional three scheduled to leave by July. Yet to be decided is whether further reductions will be made after July; President Bush on Saturday declined to promise that he will order more cuts before he leaves office in January.

A brigade usually numbers between 3,500 and 5,000 troops, depending on how it is organized.

Some military leaders have pushed for a continued withdrawal beyond July, arguing that a strained Army and Marine Corps need relief after five years of combat and multiple tours of duty with long absences from home. On the other hand, they do not want to reduce so quickly that security gains are lost.

The 2nd brigade, 82nd Airborne has been operating in northeast Baghdad since last January, when Bush announced he was sending 21,500 Army and Marine reinforcements as part of a revamped military strategy to tamp down sectarian violence. The reinforcements included five Army brigades and two Marine battalions.