Complex sentence: One panzer corps of the German 12th Army was assembled near Sofia, Bulgaria, along with one panzer corps of the First Panzer Group, and these formations were assigned the task of striking the strongest Yugoslav formations stationed along the eastern border of the country.
Split into simpler sentences: One panzer corps of the German 12th Army was assembled near Sofia, Bulgaria, along with one panzer corps of the First Panzer Group. Both formations were assigned the task of striking the strongest Yugoslav formations stationed along the eastern border of the country.