Apparently there had been a Plan Red set up in America in the 1920s and 30s for the proposed invasion by the USA of the UK and its dominions, mainly Canada. Also what the consequences would be of retaliation by the UK if such a plan ever had to come to fruition. I am heavily into the 2nd World War but now I have something new to read up about. Ye gods, I wonder what circumstances would have brought that about ! So much to learn and so little time, etc etc.
It's the kind of contingency plan Govts do to cater for any scenario that presents itself, sometimes they are more like wargaming exercises given to Officers to make them look useful.Having read a bit about it before it was never a serious plan to invade.
Years ago I worked for Leeds council and one weekend was working in the City Hall. On a desk we stumbled across the disaster plans for Leeds. Everything from flooding, through a plane crash in the city centre to a radioactive bomb. Scary stuff, but these things have to be planned for.