One development after 1914 brought a direct confrontation between the governments in Berlin and Washington: the German use of submarine warfare. Americans expected to be able to trade and travel in wartime under the protection of neutrality. Submarine warfare placed American lives in the line of fire, and the inevitable crisis with Berlin soon arrived. When a German submarine sank the British liner Lusitania on May 7, 1915, killing 128 Americans, the United States took a direct diplomatic part in the war. American pressure forced Germany to limit its use of the submarine, and the United States now assumed the role of a potential adversary if Germany's leaders renewed the submarine campaign of early 1915. Earlier frictions and irritations with Britain had arisen over American trade with the continent. The British navy, for example, diverted or detained American ships bound for Scandinavian ports to make sure they were not carrying strategic goods that could be sent on to Germany. Such concerns now faded. Americans distinguished between the minor irritations of British restrictions on the shipment of goods and the German willingness to take American lives.