In early 1915, the western Allied Powers had a dilemma: how could maritime supply lines be opened to Russia, when the German Navy ws blockading the Baltic while the Ottomans controlled entry to the Black Sea? Deciding the Ottomans were the softer adversary, a Franco-British naval task force was sent to clear the Dardanelle Straits, the gateway both to the Black Sea and Constantinople, the Ottoman capital. After earlier destroying Kum Kale and Sedd-el-Bahr, the forts guarding the mouth of the Straits, on 18 March the battle fleet advanced towards the Narrows guarded by the main Ottoman fortifications. The Allies were aware that lines of mines were strung across the straits, but not that the Ottomans had recently added a line of mines parallel to the southern shore. Under heavy bombardment, the ships banked, turned and struck the new minefield: three battleships were sunk and there were 700 Franco-British casualties.