It's well known that Hitler rose to power largely because of the desperation of the German people, who were suffering from the corruption and ineptitude of the Weimar government, hyperinflation, the threat of Communist agitators from the East, and the stupid insistence of France that Germany repay war debts they simply could not meet.
This is not to excuse what happened, but simply to point out that it's not smart to back desperate people into a corner. The allies pushed the Germans when they were hungry, weak and desperate, and then they made the opposite mistake and tried to appease them after Hitler came to power.
I used to believe the second part of your post--but I am beginning to learn things may have beem more complex then they first appear.
It is possible they allowed Germany to grow in power in the expectation it would stop the spread of communism--perhaps even that Germany and Russia would destroy each other.
Things did not turn our that way--although they almost might have; so maybe the appeasers weren't quite so foolish as they seem in hindsight.