Complex sentence: Margaret had been with her husband on the battlefield the entire time, and, after witnessing his death, she immediately took his place at the cannon, continuing to fire until her arm, chest, and jaw were hit by enemy fire.
Split into simpler sentences: Margaret had been with her husband on the battlefield the entire time, and, after witnessing his death, she immediately took his place at the cannon. She fired away until her arm, chest, and jaw were hit by enemy fire.