His biggest success of the war was the blitzkrieg of France, and it was absolute blind luck (mixed with French ineptitude and lack of preparation) that it ended up going the way it did.

One can rather say that the French too prepared for Germans trying to

reproduce the trench war stalemate of WWI on the Western front

, except when it became visible that they are not putting all their effort into that, it also became imperative for Germans to act offensively. They couldn’t afford a long standoff without France actually bleeding.

And here it became apparent that French politicians were not prepared for France actually bleeding at all.