However, as Greeks became embroiled in civil wars, the Sultan called upon his strongest subject, Muhammad Ali of Egypt, for aid. Plagued by internal strife and financial difficulties in keeping the fleet in constant readiness, the Greeks failed to prevent the capture and destruction of Kasos and Psara in 1824, or the landing of the Egyptian army at Modon. Despite victories at Samos and Gerontas, the Revolution was threatened with collapse until the intervention of the Great Powers in the Battle of Navarino in 1827. There the Egypto-Ottoman fleet was decisively defeated by the combined fleets of the Britain, France and the Russian Empire, effectively securing the independence of Greece.