Submarines and blockades dominated the war at sea in 1915. The major event was the sinking by U-20 of the Lusitania, which edged America closer to war. The Germans had announced a blockade of Britain in February, using submarines to sink and harry shipping. A British blockade of Germany was announced in March in retaliation. Using a surface fleet to stop neutral vessels, the British attempt was more successful. The British submarine force was also successful commanders leading patrols into the Black Sea and the Baltic Sea.