The first “event” that brought the Holocaust about was not really an event at all but rather a historical trend. This was the trend towards anti-Semitism in Europe in general. The fact that many Europeans were anti-Jewish long before Hitler came to power helped him come to power and helped allow him to carry out the Holocaust.
The second event was World War I and its aftermath. First, this event radicalized Hitler, who fought in the war and who believe the war was only lost by the “stab in the back.” Second, the defeat and the Treaty of Versailles made many Germans very angry and made it much easier for an extremist like Hitler to come to power.
Third, we can say that the Allies’ policy of appeasement helped bring the Holocaust about. It allowed Hitler to become more and more powerful. It also allowed him to take Austria and Czechoslovakia, making it much more likely that Jews in those countries would be killed.
Finally, the invasion of Poland and the later invasion of the Soviet Union brought the Holocaust about. This is because the majority of the Jews killed in the Holocaust came from these countries. These invasions gave Hitler control over these Jews and allowed him to have them killed.