Complex sentence: To protect their Danube frontier the Bulgarians had activated their Third Army as early as September 1915, giving its commander, Lieutenant General Stefan Toshev, almost a year to train and equip his troops.
Split into simpler sentences: To protect their Danube frontier, the Bulgarians had activated their Third Army as early as September 1915. Its commander, Lieutenant General Stefan Toshev, had almost a year to train and equip his troops.