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Because III Corps consisted of REFORGER units that were based in the CONUS. III Corps was the only American unit assigned to NORTHAG and 2nd Armored Division (Forward) was the only part of the corps permanently stationed in Europe, so the brigade commander as triple-hatted as the commander of the corps' forward headquarters and the administrative head of the US Army in Northern Germany.
The bulk of the US Army in Germany (V and VII Corps) was assigned to CENTAG in the south.