Complex sentence: During the battle of the Tannenberg line his forces were able to withstand a superior Soviet force with only 7 tanks remaining: his unit then withdrew with the rest of Army Group North to the Courland Peninsula.
Split into simpler sentences: During the battle of the Tannenberg line his forces were able to withstand a superior Soviet force with only 7 tanks left. His unit then withdrew with the rest of Army Group North to the Courland Peninsula.