In the late 20's, early 30's, the United States developed War Plan Red. War Plan Red was our plan if we went to war with Great Britain. My question is why did we feel the need to develop a plan for war against Great Britain in the 20th century. Was there underlying issues that the government believed could develop into a conflict? I thought we were really close with Great Britain after WW1. Or was this just one of the thousand of hypothetical plans that the government made over our history.
Very much the last, I think: military planning staffs' job was to have plans for conceivable eventualities, without any one scenario necessarily being thought likely. The US had a range of such cases, named in this period for colours (Japan was "Orange", Germany "Black"). It's still puzzling that this one was still being developed so late given the extent of co-operation, rather than just gathering dust under a heap of paper. There were no issues likely to result in armed conflict, and British chiefs had abandoned all thought of even defending Canada against the US back in 1908 (But keep that under your hat).
I wish I could find more of the colour plans, but I've read Red.
It was good practice in planning, and all I've ever seen is the plan for holding Canada, not anything about invading Wales.
There always was a small possibility of either Communists or fascists coming to power in Britain. So whether Operation Sea Lion worked or the home-grown dictator took over, it would be a nice little luxury to have a plan for trouble on the northern border. Sort of like keeping the extra pack of TP in the back of the broom closet for a real emergency.
None of those were meant as serious "first strike" plans for US conquering the world.