At that point, the three Great Powers—Russia, Britain and France—decided to intervene, sending their naval squadrons to Greece in 1827. Following news that the combined Ottoman–Egyptian fleet was going to attack the island of Hydra, the allied fleets intercepted the Ottoman navy and won a decisive victory at the Battle of Navarino. In 1828 the Egyptian army withdrew under pressure of a French expeditionary force. The Ottoman garrisons in the Peloponnese surrendered, and the Greek revolutionaries proceeded to retake central Greece. Russia invaded the Ottoman Empire and forced it to accept Greek autonomy in the Treaty of Adrianople (1829). After nine years of war, Greece was finally recognized as an independent state under the London Protocol of February 1830. Further negotiations in 1832 led to the London Conference and the Treaty of Constantinople; these defined the final borders of the new state and established Prince Otto of Bavaria as the first king of Greece.