Complex sentence: By September 1914 the Russians had defeated the Austro-Hungarians in the Battle of Galicia, the Austro-Hungarian retreat isolated their fortress of Przemyśl, which was besieged by the Russian Eighth Army.
Split into simpler sentences: By September 1914 the Russians had defeated the Austro-Hungarians in the Battle of Galicia. The Austro-Hungarian retreat isolated their fortress of Przemyśl, which was besieged by the Russian Eighth Army.