Last week, Mao Ning, head of China’s foreign ministry information department, posted a blurry black-and-white clip of a moment in history. In 1953, Chairman Mao made a defiant speech of resistance to what he called US aggression in Korea. Kim Il-sung, the North Korean leader and founder of the Kim dynasty, now in its third generation, had invaded US-backed South Korea. When Kim’s attempt to unite Korea by force appeared to be failing, China threw nearly 3 million “volunteers” into the war and succeeded in fighting to the stalemate that has prevailed ever since. There was no mistaking the symbolism...