This is always a hotly contested question, no matter the war. Thucydides teaches that the real cause was not the declaration of war by Sparta that officially started the Peloponnesian War, but the imbalance of power created by Athenian aggression over time relative to the Spartan reticence to act. Thucydides is teaching that wars are determined years in advance. It is the creation of the conditions of war that we need to look at to explain any given war. … More Who Started the War? Thucydidean Realism 2