The treaty of Versailles was a peace treaty after WWI. This treaty made Germany pay for the war. They did this by forcing them to pay for reparations, give up their military and take full responsibility for the war.
We must destroy the Jews!
The great depression caused the Germany economy to collapse. This made all the Germans very desperate which helped Hitler receive more power.
People were starving, broke, and lost almost everything!
Invasion of Poland
Hitler took advantage of the Germans anger and told them that he would fix all their problems. So he blamed the Jews for everything and decided that he needed to kill all of them.
Many countries didn't agree with this idea, but they wanted to avoid problems and avoid another war. So they appeased Hitler and allowed him to take over other places.
In September of 1939, Hitler invaded Poland and that was the beginning of WW2.