During the last few years of World War II, the United States and the Soviet Union - both long weary of the other - became unlikely allies against Adolf Hitler's takeover of Eastern Europe. Following the defeat of German in 1945, however, the wartime allies became mortal enemies, locked in a global struggle to prevail militarily, ideologically, and politically in a new "Cold War." To learn of the other side's military and technical capabilities, their actions and intentions, both sides used...