Dardanelles (därdənĕlzˈ) [key] or Çanakkale Boğazi chänäkˈkälĕ bōäzŭˈ, strait, c.40 mi (60 km) long and from 1 to 4 mi (1.6 to 6.4 km) wide, connecting the Aegean Sea with the Sea of Marmara and separating the Gallipoli peninsula of European Turkey from Asian Turkey. It was called the Hellespont in ancient times and was the scene of the legend of Hero and Leander. Its modern name is derived from Dardanus, an ancient Greek city on its Asian shore. Controlling navigation between the Black Sea and the Mediterranean, the Dardanelles and Bosporus straits have long been of immense strategic and commercial importance. Ancient Troy prospered at the western entrance to the Hellespont. Xerxes I crossed (c.481 B.C.) the strait over a bridge of boats, as did Alexander the Great in 334 B.C. Throughout the existence of the Byzantine and Ottoman empires the Straits were essential to the defense of Constantinople (İstanbul). By 1402 the Dardanelles were under the control of Ottoman Sultan Beyazid I. Muhammad II began (15th cent.) to fortify the passage, which, with brief interruptions, has remained in Turkish hands until the present. Russian expansion along the Black Sea (from the 18th cent.) and the resulting weakening of the Ottoman Empire became of great concern to the Western powers (see Eastern Question), notably England and France, which from 1841 joined forces to prevent Russia from gaining control over, or special rights in, the Straits. In 1841, England, France, Russia, Austria, and Prussia agreed to close the Straits to all but Turkish warships in peacetime. This convention was formally reaffirmed by the Congress of Paris (1856) at the end of the Crimean War and, theoretically at least, remained in force until World War I. Early in 1915 an Anglo-French fleet, commanded first by Admiral Carden and later by Admiral Sir John de Robeck, sought unsuccessfully to force the Dardanelles and take Constantinople. A second attempt, known as the Gallipoli campaign, was also unsuccessful, but after the final Turkish collapse an Allied fleet passed (Nov., 1918) the Straits and occupied Constantinople. The Treaty of Sèvres (1920) with Turkey internationalized and demilitarized the Straits zone, but it was superseded by the Treaty of Lausanne (1923). The zone was restored to Turkey, but was to remain demilitarized; the Straits were to be open to all ships in peacetime and in time of war if Turkey remained neutral; if Turkey was at war, it could not exclude neutral ships. Secretly, however, Turkey soon began to refortify the zone, and in 1936, by the Montreux Convention, it was formally permitted to remilitarize it. Turkey has maintained the right to restrict the access of ships from non-Black Sea states.