Complex sentence: Louis failed to regain the championship when he returned to the ring in 1950, and when Rocky Marciano knocked him out in 1951, the man who had been called the Brown Bomber was finished.
Split into simpler sentences: Louis failed to regain the championship when he returned to the ring in 1950 and when Rocky Marciano knocked him out in 1951. The man who had been called the Brown Bomber was finished.