Complex sentence: The corps area became a rear area for the corps, responsible for training and equipping replacement troops and other duties.Also, to the regular army corps, reserve corps were formed on mobilization in 1914, and were joined by additional corps as World War I progressed.
Split into simpler sentences: The corps area became a rear area for the corps, responsible for training and equipping replacement troops and other duties. In addition to the regular army corps, reserve corps were formed on mobilization in 1914, and were joined by additional corps as World War I progressed.