I don't know if I'll type you a whole essay... but!
There are a lot of factors that contribute to the German army's early victories. One of the biggest is the amount of modernization that the German nation went through, they developed new weapons and vehicles that would help supplement their new Blitzkrieg tactics. Another factor is the fact that the Allied Nations (Britian and France) were doing their best to appease the aggressive German nation, they allowed them to breach the confines of the Treaty of Versailles and invade neighboring nations. Another factor is that Hitler had major ambitions and dreams for what he thought was possible and was in a position to have perhaps some of the greatest engineers this world has known create his crazy ideas, many of which crafted modern warfare.