A war leader travels across a vast ocean – at Christmastime and under a heavy cloak of security – to thank, cajole and strengthen ties with the United States. Historical analogies can be deadly in foreign policy. If everything is viewed through the prism of Munich in 1938, the risk is of future Vietnams. But it was difficult not to see in Volodymyr Zelensky a flash of Winston Churchill, who sailed to the US in December 1941, within days of the Japanese attack on Pearl Harbor, to address a joint session of Congress. Zelensky’s was a visit bursting with symbolism....