Complex sentence: The end of the war hit BMW particularly hard, since the BMW IIIa aero-engine was the only product the company was building in 1918, and suddenly there was no more demand for aircraft engines.
Split into simpler sentences: The end of the war hit BMW particularly hard, since the BMW IIIa aero-engine was the only product the company was building in 1918. And suddenly there was no more demand for aircraft engines.