... In the late 1940s, Winston Churchill wrote a memoir, “The Second World War,” six volumes that helped win him a Nobel Prize in Literature while burnishing his glory. Franklin Roosevelt meant to write his own account, collecting papers and setting up the first presidential library. But, by dying in office, Roosevelt missed the chance to toot his horn as loudly as his wartime partner. Churchill was able to play down or obscure his “often suspect” military leadership, writes Nigel Hamilton in “The Mantle of Command,” while Roosevelt’s deft but opaque role as commander in chief has been overshadowed or overlooked in many military histories. In his fast-paced, smartly observed recounting of Roosevelt’s first year as war leader, Hamilton means to set the record straight...