Complex sentence: The VII Corps in the south was reinforced by two divisions, a Cavalry division and the First Group of Reserve Divisions and became the Army of Alsace to relieve the First Army of concern about Alsace during the operations in Lorraine.
Split into simpler sentences: The VII Corps in the south was reinforced by two divisions, a Cavalry division and the First Group of Reserve Divisions. The corps was renamed the Army of Alsace, to relieve the First Army of concern about Alsace during the operations in Lorraine.