Complex sentence: The Seventh Army was transferred to the 12th Army Group, and the French First Army reverted to national control, leaving the 6th Army Group with little to do, although Devers acted as commander of the 12th Army Group in Bradley's absence.
Split into simpler sentences: The Seventh Army was transferred to the 12th Army Group, and the French First Army reverted to national control, leaving the 6th Army Group with little to do. Devewrs acted as commander of the 12th Army Group in Bradley's absence.