The first, and arguably most important, event that led to the Holocaust was the rise of Fascism in Germany. Throughout the 1920s, Adolf Hitler campaigned openly on a platform of anti-semitism, and the Nazis inaugurated his regime with a one-day boycott of Jewish-owned shops in April of 1933.
With the 1934 death of President Paul von Hindenburg, one of the last restraining forces on Nazi anti-semitism was removed. As president and chancellor, Hitler oversaw the passage of the infamous Nuremberg Laws of 1935. These laws restricted Jewish life in Germany to an unprecedented degree. As they took effect, Jewish civil servants were turned out of their jobs, Jewish doctors and lawyers saw their practices restricted, Jewish families were prohibited from employing non-Jewish domestic servants and Jewish men and women were required to take the middle names "Israel" or "Sarah."
The next major wave of repression came in 1938, with the murder, by a Jewish assassin, of a German diplomat. The Nazis used this as a pretext to further restrict Jewish life and marginalize their target population. Synagogues were burned during the pogrom that followed, and thousands were arrested. As World War II began, German execution teams spread across first Poland, then Russia, executing "undesirables," mostly Communists and, eventually, Jews. In 1942, the killing program was formalized as Operation Reinhardt, which operated until late in 1944, when the Red Army overran the death camps.