The Peloponnesian league was led by the city-state Sparta and included most of the city-states of the Peloponnesian Peninsula. It was a league in direct confrontation/rivalry to Athen's Dellian League. The league was simply a defensive pact between all the member states and Sparta, not with each other. So member states could war against each other if they chose to. Unlike the Dellian League, member states di not have to pay tribute to Sparta, but only in times of war, along with a 3rd of the military of each state. In the Peloponnesian Wars, it was the Peloponnesian League that eventually won against Athens.