Would a Chinese leader barely in control of his own country after a long civil war dare attack a superpower that had crushed Japan to end World War II five years earlier by dropping atomic bombs on Hiroshima and Nagasaki? As American troops pushed North Korean forces toward the Chinese border in 1950, Gen. Douglas MacArthur could not imagine so. But Mao Zedong did. MacArthur was dumbstruck. Chinese forces rapidly beat American troops back to the line that had divided North and South Korea when the war began. That thirty-eighth parallel continues to mark the border between the two Koreas today. By the time the war ended, nearly three million had perished, including thirty-six thousand American troops.

Similarly, in 1969, Soviet leaders could not imagine that China would react to a minor border dispute by launching a preemptive strike against a power with overwhelming nuclear superiority.