Peter Harmsen is a journalist and the author of Fury and Ice: Greenland, the United States and Germany in World War II. The historically minded among us may have sensed a certain déjà vu this weekend when U.S. President Donald Trump talked to NBC News about the role military force could play in gaining control of Greenland, currently an autonomous territory of long-time ally Denmark: “I don’t take anything off the table.” After all, in 1940, when the Americans were slowly waking up from their isolationist slumber to side with the Western democracies in the struggle against fascism, they took...