The main motivating factor was Hitlers propaganda and supremacists ideology in a time of economic misery
Hitler had exploited the misery and suffering in post war German, blamed the oppressive terms of the Versailles on the entente, and aroused supremacists thoughts in the Aryan race against the Jews who they accused of being a stab-in -the back. This had served to sediment an already hungry and angry young men to join the war.