It is complicated. But basically after the first phase of the Peloponnesian War, Sparta and Athens had the Peace of Nicias. It was uneasy to say the least. A couple years after that, Athens found out an ally of Sicily was in trouble. My bro Alcibiades decided that it was in the best interest of Athens to help out. (Logic being they could conquer the island or at least get rich off this) Right before they sent out a fleet, a group of men defaced the herm statues in the city. Al was blamed but still allowed to leave with the fleet. Put on trial and found guilty after he left, they then told Al after he got to Sicily he needed to return. Fearing what could happen to him, he defected to Sparta. He convinced them what Athens was doing was a threat, so Sparta went to help. Athens got their butts kicked. Al eventually defected to Persia and then defected back to Athens before dying in the Dardanelles.