On the ground floor of the White House is the Map Room, so-called because it was here that Franklin Roosevelt used to get his briefings on the progress of World War II. Over the mantel is the last map FDR saw before his death. It shows American, British, and Soviet troops racing toward Berlin. It also shows a frightening concentration of German forces in the Nazis’ last redoubt, the mountains of Bavaria. We now know of course that this last redoubt did not exist. American intelligence had been deceived. And it’s possible that policymakers also deceived themselves. Roosevelt, for reasons...