 With dwindling revenues from European sponsors no longer willing to support a costly and meaningless organization, the knights turned to policing the Mediterranean from the increased threat of piracy, most notably from the threat of the Ottoman-endorsed Barbary pirates operating from the North African coastline.Boosted towards the end of the 16th century by an air of invincibility following the successful defence of their island in 1565 and compounded by the Christian victory over the Ottoman fleet in the Battle of Lepanto in 1571, the knights set about protecting Christian merchant shipping to and from the Levant and freeing the captured Christian slaves who formed the basis of the Barbary corsairs' piratical trading and navies.This became known as the "corso".