Complex sentence: He defeated Marshal Macdonald at the Katzbach, and by his victory over Marshal Marmont at Möckern led the way to the decisive overthrow of Napoleon at Leipzig, which was taken by Blücher's own army on the evening of the last day of the battle.
Split into simpler sentences: He defeated Marshal Macdonald at the Katzbach, and by his victory over Marshal Marmont at Möckern led the way to the decisive defeat of Napoleon at Leipzig. Leipzig was taken by Blücher's own army on the evening of the last day of the battle.