They didn't, it was just a subsect of War Plan Red. The US had invasions of Canada, New Zealand, India, and Ireland as part of War Plan Red as well IIRC. Canada was "Crimson".
I know that Prohibition nearly brought the US and Uk to war in the 20s and early 30s. Since Japan was still allied with Britain at that time, and France woudl have also gone to war with the USA, I wonder how a Japanese attack on Pearl Harbour would have been carried out in the 1930s.
The British and Canadians allowed alcohol to illegally flow into the USA while the USA let the Fenian Brotherhood launch terrorist attacks on British interests in Canada.
And I would imagine such war plans were developed when there was the possibility of Oswald Mosely become Prime Minsiter, and the British Union of Fascists taking power.
I have seen a few alternate history scenarios on other AH sites of how WWII might have went, if Moseley had come to power, with no realiable ally in Europe. I think the B-36 would have been delvoped earlier, to allow for long range bombing runs launched from bases in the USA.