While there are only 50,000 troops left in Iraq, and none of them are, technically, combat troops, most of them are. That's because most of the troops left in Iraq are there to provide training and advice for Iraqi security forces. Thus the U.S. has eleven combat brigades in Iraq. Seven of them are regular combat brigades, retrained and reorganized for the "training and advice" job. Two of the brigades are reserve infantry units, to provide security for bases. Another two are aviation brigades, to provide transport and attack helicopters as needed for advising and defense.
The seven advisor brigades have most of their regular equipment, and can be quickly switched to the purely combat role. These Advise and Assist Brigades (AABs) were developed several years ago. They are regular combat brigades that leave many of their junior troops back home, leaving a force heavy on officers and NCOs, who are best equipped to advise and assist local troops and commanders. The troops in these brigades received some extra training for their advising and training tasks. This works, as American troops have been advising and training the Iraqis. Meanwhile, there are another 10,000 U.S. troops in Kuwait, mostly performing support roles.