Article: Lack of political unity within Greece resulted in frequent conflict between Greek states. The most devastating intra-Greek war was the Peloponnesian War (431–404 BC), won by Sparta and marking the demise of the Athenian Empire as the leading power in ancient Greece. Both Athens and Sparta were later overshadowed by Thebes and eventually Macedon, with the latter uniting the Greek world in the League of Corinth (also known as the Hellenic League or Greek League) under the guidance of Phillip II, who was elected leader of the first unified Greek state in history.

Question: What often caused strife between Greek states?
Lack of political unity