“...how, despite all the prayers, fasting and sacrifice, did God allow just one man, Hitler, to come to power and doom 50 million mem women and children...”
This is the mystery of iniquity - how does God allow evil? The answer lies in the fact the devil entered our world through original sin, and has power in the world to influence free will. The reason that man is like God is because God created him with the intellectual powers of reason and free will. Each individual man can and does have the power to choose evil.
If God didn’t allow us free will to choose evil, we would be robots and not humans.
Each individual man has the power to choose good for himself, and that is where fasting comes in. Fasting in solidarity with Christ’s fasting (the 40 days in the desert, for example), helps us to remain close to Christ and and like Him, reject the devil.
The devil has influence over this world and is it very easy for humans to fall into his and the demons’ power.
One could make the argument that the fasting allowed the Allies to win in the end, so the western world is not speaking German.