The holocaust occurred during World War II in Germany. The German leader, Adolf Hitler, attempted to exterminate the Jewish people by confining them to concentration camps and then killing them systematically using gas chambers, working them to death with no food or medical care and mass murder and burial in mass graves. Genocide was practiced on a scale never before seen. Not only Jews were subjected to this “cleansing" of the world, but other "inferior races" as proclaimed by the Nazi regime. Homosexuals, blacks and any races other than the pure Aryan race were seen as being the cause of the world’s problems by the Nazi's during this time. Although there were numerous other factors that helped to cause the Holocaust, these are the major reasons.

The first cause of the Holocaust was the climate that existed in Germany between WWI and WWII. Following the conclusion of WWI, Germany’s defeat was complete. The powers that won felt that Germany should make reparations to the countries that it had invaded, the peoples that had been conquered and to the Allied Forces.
In other words, Germany was forced to accept the blame for starting WWI. The reparations destroyed the German economy. Unemployment was running rampant. Perhaps the most important factor was that the Germans couldn’t understand how they had lost the war and it damaged their pride.
The treaty of Versailles was another cause of the Holocaust. The demands placed on Germany in the treaty required the country to give lands back to the Allied forces or back to the people that had populated them, such as East Prussia which was supposd to be returned to Poland. In addition to this provision, the heartland of Germany also had to accept military forces being stationed throughout Germany to guard against another war for the following fifteen years. The central part of Germany was demilitarized.
Anti-Semitism was encouraged by Hitler in order to give Germans a scapegoat for the troubles the German people were having. It was not a religious form of anti-semitism, but was a racial form instead. The Jews were accused of not being as evolved as the other races and Hitler claimed they were responsible for the defeat during the war.
The economic troubles of Germany were blamed on the Jews. They were accused of taking all the money for themselves. The Nazi party generated extensive propaganda to this end. As anti-Semitism grew within the population, the things done to the Jews by Hitler and his army began to be widely seen as acceptable. Herding Jews into slums, burning and taking their businesses and finally sending them to camps all became common.
Finally, Adolf Hitler himself was a cause of the Holocaust. Hitler was obsessed with the destruction of the Jews. Documents that have become accessible since WWII seem to point at the fact that he probably suffered from syphilis acquired as a youth in Austria and, left untreated, resurged later in life.
Syphilis has outward symptoms for only about nine months but then all symptoms disappear and it lays dormant in the body for anywhere from one to fifty years. When it becomes active again, it can take many courses. Some are affected in the nervous system while some are affected in the brain. By examining his history and young life, it appears that he may have caught it in Vienna around 1908. It then disappeared and only reemerged around 1935 as it began the tertiary stage of the disease.
The effects of syphilis on the brain can include paranoia, megalomania, loss of a sense of reality and loss of any moral senses. All of these can be found in his behavior during the later part of his life. Syphilis is also characterized by fits of anger.
As an adherent to social Darwinism, Historians believe Hitler may have attempted to solve his inner struggles caused by syphilis by practicing extreme brutality and senseless killing. Extermination of the Jews and “inferior races” became an obsession with him.