The naval Battle of Cynossema took place in 411 BC during the Second Peloponnesian War. In the battle, an Athenian fleet commanded by Thrasybulus and Thrasyllus, although initially thrown on the defensive by a numerically superior Spartan fleet, won a narrow victory. This victory had an impact out of proportion to its tactical significance, coming when Athens' traditional democratic government had been replaced by an oligarchy and an Athenian defeat could have ended the war. The newly confident Athenian fleet proceeded to win two more victories in the Hellespont in quick succession, the second being the dramatic rout at Cyzicus, which ended the immediate Spartan threat to Athens' Black Sea lifeline.