There were many factors that led to the WW2 Some of which are-
The treaty of versalies- The treaty of versalies was signed to end the world war 1 led the Germans to feel victimised and they became aggressive leading to start a war with US.
The great depression- It caused widespread unemployment and major inflammation in Germany which facilitated the Hitler's career in beginning.
Nazi's ideology was taking a high toll and the Germans felt betrayed and hence developed a feeling of us or them.