The task of developing a comprehensive listing of US forces present in the area is particularly difficult as the number of units rotating in and out of Southwest Asia are substantial. The events of Sept. 11, 2001 and the Global War on Terrorism has made such an effort significantly more difficult as the military seeks to improve operational security (OPSEC) and to deceive potential enemies and the media as to the extent of American operations. The significant number of troops that have been mobilized or activated in the United States for unspecified duties, whether they be in support of Iraq Freedom or in support of homeland security also complicates matters as it is not entirely clear what units are going where. This is further compounded by the increasing reliance on the part of the Department of Defense on the mobilization of small-sized units to fill its manpower needs. These can range from individual companies, batteries, to even smaller-sized groups of troops rather the mobilization of entire battalion-sized units.
This being the case, mistakes, misidentifications, or ignorance regarding specific units being in the region, or not in the region will occur.
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This figure may, however, have been an undercount of actual in-country troop numbers, as Special Forces have been reported to generally be excluded from troop totals. As such, the total figure of US troops in Iraq may be higher than the official count of ~150,000 by multiple thousands. One such Special Forces unit, the 10th Special Forces Group deployed to Iraq in late-2004, for an undetermined length of time.
go here for an overview:
http://www.globalsecurity.org/military/ops/iraq_orbat.htmTry here:
http://www.globalsecurity.org/military/ops/iraq_orbat_toe.htmAnd especially here (look at the maps):
http://www.globalsecurity.org/military/facility/iraq.htmOn Edit:You definitely want to look here-
http://www.globalsecurity.org/military/facility/iraq-2005.htm