New ideas of security spelled the end of piracy on the Mediterranean Sea during the nineteenth century. As European states ended their military conflicts and privateering wars against one another, they turned their attention to the 'Barbary pirates' of Algiers, Tunis and Tripoli. Naval commanders, diplomats, merchant lobbies and activists cooperated for the first time against this shared threat. Together, they installed a new order of security at sea. Drawing on European and Ottoman archival records – from diplomatic correspondence and naval journals to songs, poems and pamphlets – Erik de Lange explores how security was used in the nineteenth century to legitimise the repression of piracy. This repression brought European imperial expansionism and colonial rule to North Africa. By highlighting the crucial role of security within international relations, Menacing Tides demonstrates how European cooperation against shared threats remade the Mediterranean and unleashed a new form of collaborative imperialism.