Complex sentence: The Athenian coup of 411 BC was a revolutionary movement during the Peloponnesian War between Athens and Sparta that overthrew the democratic government of ancient Athens and replaced it with a short-lived oligarchy known as The Four Hundred.
Split into simpler sentences: The Athenian coup of 411 BC was the result of a revolution that took place during the Peloponnesian War between Athens and Sparta. The coup overthrew the democratic government of ancient Athens and replaced it with a short-lived oligarchy known as The Four Hundred.