Within a generation the attitude and policy of the United States toward alliances have undergone a revolutionary reversal. The nation has passed from its traditional suspicion and fear of "entangling alliances" to a policy that heavily stakes its security and interests on the co-operation of other powers. In World War I the U.S. Government cautiously defined its relationship with the powers allied against Germany as that of an Associated Power. In World War II, though last to join the Grand Alliance, it virtually integrated its resources with those of the British Commonwealth and coordinated its strategy and war aims with the British and the USSR in the most powerful wartime partnership ever forged. Since 1945 it has emerged as the leader in the North Atlantic Treaty Organization (NATO) and has diligently sought allies and built up alliances all over the troubled world. The climax of its most intensive experience with coalition strategy came in the phase of World War II described in this volume, which should therefore have a special interest for all who are concerned with the implications of the revolution in U.S. foreign policy that has taken place in the twentieth century.