Why were Britain and France so unprepared for World War 2?

Answer 1

The threat of War for the US wasn't apparent until at least the beginning of 1940, as an example, there were only 800 Officers in the Marine Corps at that time. That number increased to 37,067 in 1945. Wartime production wasn't placed in gear until the German attack on Poland in 1939.

Answer 2

Because they had hoped that disarmament & appeasement would supply a lasting peace: Naive in hindsight, but at the time most people accepted it as a good idea. If you accept that most people had a very fresh rememberance of WW1 who would wish for a repitition ? and then, in the event it turned out to be even more destructive. But people generally hoped for peace, the thinking being that if countries prepared for war then it would be inevitable. Also, of course, hindsight again, no one really believed that Hitler was anything like what events showed him to be. Indeed many, especially in the British Establishment, thought that he was just the ticket ! As for my opinions of the conduct of the French in the ensuing years of WW2 it might be best if I say nothing.......