At the outbreak of World War II, both the United States and Turkey took neutral positions. While the United States joined the war in 1941 after the Japanese attack on Pearl Harbor, Turkish leaders tried very hard to maintain their neutrality until the last days of the war. These efforts defined Turkey's relations with the Allies in general, and with the United States in particular. During the war, the United States acknowledged Britain's preeminent position in shaping relations with Turkey. In the initial stages of the war, the British and the Americans concurred on the strategy of engaging in economic warfare through Lend-Lease aid and preemptive purchases of strategic materials, in hope of keeping the Turks out of the German sphere of influence. Later on, however, their views diverged on the issue of Turkish neutrality. While the British insisted on pressing for Turkish entry into the war, the Americans, who preferred to focus on the cross-Channel operation, opted for Turkish neutrality in order not to divert supplies from the European front.