Anyone who read Mein Kampf could see what was coming.
No statesman of the time was more clear-eyed and far-sighted than Winston Churchill. I have here Churchill's memoirs, written after the war, in which he discusses Mein Kampf as some length.
Mein Kampf clearly laid out Hitler's plans for conquest in Eastern Europe, but Churchill's review mentions nothing about genocide, mass murders or other atrocities.
So, we have to ask, if this portion of Hitler's agenda escaped even Churchill's scrutiny, how much of it could lesser men have predicted, back in 1939?
For what it's worth, my opinion: The whole idea of "appeasement" was to let the Germans have what they said they wanted, but peacefully. In time though, it finally became clear the peace was not an option for Hitler -- then appeasement got replaced by opposition, and soon, by war.