...it hasn't been secret for more than 30 years, but it did exist. It was approved by the War Department in 1930 and updated in 1934 and 1935.
War Plan Red was actually designed for a war with England. In the late 1920s, American military strategists developed plans for a war with Japan (code name Orange), Germany (Black), Mexico (Green) and England (Red). The Americans imagined a conflict between the United States (Blue) and England over international trade: "The war aim of RED in a war with BLUE is conceived to be the definite elimination of BLUE as an important economic and commercial rival."
Churchill's third speech to Parliment in 1901 was on the prospect of War with the United States resulting from the boundary dispute between British Guiana and Venezuela. War between the two countries wasn't unthinkable.
Evil would be the counsellors, dark would be the day when we embarked on that most foolish, futile, and fatal of all wars--a war with the United States. If such a fit of madness were to occur, he warned, both nations, having long enjoyed a glorious immunity from the curse of militarism, would be similarly placed, and no decisive events could be looked for until the war had been in progress for a year or two and enormous armies had been raised on both sides.
--page 45 of Martin Gilbert's Churchill and America
Seems far fetched today but America had huge German and Irish populations, hostile to Britian and not afraid to say so. There was evil counsel given and it was given right through 1941 from fellows like Lindbergh building on that widespread hostility.
He did not see the conflict as basically a war for democracy or morality. He was skeptical of the ideology and moral righteousness of the British and French.
Fits of madness have happened.