In late September the U.S. Army announced it was sending a division headquarters (about 500 troops) to Iraq, the first such unit to be in Iraq since 2011. Actually only about 200 of those troops will be in Iraq, the rest will be at other bases in the region. About a third of the troops in Iraq will be up north in the Kurdish controlled area while the rest will be near Baghdad.
The U.S. Army introduced new divisional and brigade organizations shortly after 2001. These reforms made the brigades, not the divisions, the primary combat unit. The new divisions still controlled many major support units. All these support units, including the division headquarters, were redesigned to be capable of operating independently, which is why just a division headquarters is being sent to Iraq. The reorganization resulted in a lot of new names for components of the division. But the division headquarters remained a battalion size unit, although with a slightly different mix of specialist sub-units. Other parts of the division, especially the support units, also went through some changes. There are now five types of “sustainment” (support) brigades. These are; Artillery Fires (cannon and rockets), Aviation (helicopters and UAVs), RSTA (Reconnaissance, Surveillance and Target Acquisition, formerly reconnaissance units, but RSTA includes UAVs and intelligence troops as well), Maneuver Enhancement (division headquarters), and Sustainment (the old Division Support Command, but smaller because many support troops are now assigned to brigades) brigades.
The new division headquarters was designed for what the one from the 1st Infantry Division is being sent to Iraq for; coordinate the activities of a bunch of support and combat brigades. The new division organization was designed to be very flexible and that is the watchword in Iraq this time around. The 1st Infantry Division headquarters will coordinate operations of a lot of small combat units, contractors and support organizations.
The new brigades have more support units permanently attached and can be more easily sent off to fight by themselves. In the past, doing this involved quickly adding a lot of support units to the brigade. But the new organization makes small support units part of the brigades and, more importantly, the brigades train using these support units and learn to work well with them. The divisions still exist but operate more like the corps has for the last two centuries (coordinating the actions of a few divisions and only having a few support units under its command).
The reorganized divisions originally had four of the new brigades but can control more (or less) in action. The recent budget cuts and combat experience have resulted in a return to three combat brigades per division. Each of the new brigades (or BCTs, for Brigade Combat Teams) has 3,500-4,000 troops (depending on the type). There are three types of BCTs: light (infantry, including paratroopers), heavy (mechanized, including tanks), and Stryker (mechanized using wheeled armored vehicles). This larger number of combat brigades is achieved by reorganizing the combat units of each division into four brigades, instead of the current three. There are several independent brigades as well.