Germany's rapid success in the first months of the war was largely due to the

The correct answer is use of the blitzkrieg, which caught countries unprepared.

They practiced blitzkrieg which was a form of warfare where they would just take a huge amount of tanks and air fighters and completely bomb and run over the country that is being attacked without giving them the opportunity to defend themselves or to prepare for the attack. This was very successful but eventually they were stopped when they went deep into Russia.