Because FDR had already planned to go to war against Germany, and had funded its enemies, and attacked German ships in the Atlantic.
"on December 4, 1941, that the pro-fascist Chicago Tribune and its' sister publication the Washington Times-Herald printed the plans for the top-secret Rainbow 5 Plan."
"In Hitlerís speech declaring war against the United States on December 11, 1945, the final straw he listed was as follows.
"With no attempt at an official denial there has now been revealed in America, President Rooseveltís plan by which, at the latest in 1943, Germany and Italy are to be attacked in Europe by military means."
In this excellent book, the Rainbow 5 plan and the leak of it are described, as well as considerable evidence pointing to FDR himself as a source of the leak. This book is the best one to read that I have found regarding FDR and WWII. It is also revealed that communist sympathizers and even spies were in the highest levels of the FDR Administration.
[ Acclaimed historian Thomas Fleming brings to life the flawed and troubled FDR who struggled to manage WWII. Starting with the leak to the press of Roosevelt's famous Rainbow Plan, then spiraling back to FDR's inept prewar diplomacy with Japan, and his various attempts to lure Japan into an attack on the U.S. Fleet in the Pacific, Fleming takes the reader inside the incredibly fractious struggles and debates that went on in Washington, the nation, and the world as the New Dealers, led by FDR, strove to impose their will on the conduct of the War. Unlike the familiar yet idealized FDR of Doris Kearns Goodwin's No Ordinary Time, the reader encounters a Roosevelt in remorseless decline, battered by ideological forces and primitive hatreds which he could not handle and frequently failed to understand-some of them leading to unimaginable catastrophe. Among FDR's most dismaying policies, Fleming argues were an insistence on "unconditional surrender" for Germany (a policy that perhaps prolonged the war by as many as two years, leaving millions more dead) and his often uncritical embrace of and acquiescence to Stalin and the Soviets as an ally. For many Americans, Franklin Delano Roosevelt is a beloved, heroic, almost mythic figure, if not for the "big government" that was spawned under his New Deal, then certainly for his leadership through the War. The New Dealers' War paints a very different portrait of this leadership. It is sure to spark debate. ]