Complex sentence: Although initially successful, the offensive was brought to a halt after Bulgaria's attack on Dobruja, and a successful German and Austro-Hungarian counterattack after September 18 eventually forced the Romanian Army to retreat back to the Carpathians by late October.
Split into simpler sentences: Although initially successful, the offensive was brought to a halt after Bulgaria's attack on Dobruja. Coupled with a successful German and Austro-Hungarian counterattack after September 18, the Romanian Army was eventually forced to retreat back to the Carpathians by late October.