Since losing against Barbarossa Hayreddin's Ottoman fleet at the Battle of Preveza in 1538 and the disastrous expedition of Emperor Charles V against Barbarossa in Algiers in 1541, the major European sea powers in the Mediterranean, Spain and Venice, felt more and more threatened by the Ottomans and their corsair allies. Indeed, by 1558 Piyale Pasha had captured the Balearic Islands and together with Turgut Reis raided the Mediterranean coasts of Spain. King Philip II of Spain appealed to Pope Paul IV and his allies in Europe to organize an expedition to retake Tripoli from Turgut Reis, who had captured the city from the Maltese Knights in August 1551 and had subsequently been made Bey (Governor) of Tripoli by Sultan Suleiman the Magnificent.