In the 1930s, Adolf Hitler and Benito Mussolini met several times in each other’s countries to pledge their unified opposition to the post-World War I liberal international order. The two dictators committed Nazi Germany and Fascist Italy to a “common destiny”: to create “a New Order” in Europe, an “axis” around which the fate of the continent would revolve. Undeterred by the West, they then unleashed the horrors of World War II. Vladimir Putin and Xi Jinping, today’s most threatening world leaders, have also engaged in a series of exchange visits, the latest of which is happening this week with...