What caused the holocaust, what were its roots?

The roots of the Holocaust lay in the antisemitism of ancient times. Long before the birth of Jesus, Jews were attacked and oppressed because of their unwillingness to accept other forms of belief pressed upon them by hostile forces, including, to certain degree, the formidable forces of ancient Rome. Judaism was the first of the major monotheistic religions, and the polytheistic nature of the ancient Romans and Greeks could not both countenance this divergence in belief systems and maintain legitimacy grounded in claims of divine rule. Now, that is, obviously, ancient history, but the antisemitism manifested throughout history led directly to the Holocaust by laying the seeds of hatred towards Jews, invariably grounded on crude stereotypes about Jewish religious practices. 

Antisemitism exploded as a force with the life and death of Jesus, as Jews bore the blame for the Crucifixion because of their unwillingness, again, to accept alternative approaches to God. Jewish adherence to its underlying principles discounted the notion of a human God placed upon Earth in the guise of Jesus, as worship of Jesus ran counter to the monotheistic nature of Judaism. As Christianity spread across Europe, hatred of Jews spread with it, with the Crusades and Inquisition both among the more spectacular manifestations of antisemitism predating the Holocaust of the middle of the 20th century. It would be the development of the political thoughts of Adolf Hitler during the early 20th century that would presage that most horrific of all developments in human history.

Hitler, as we know, had served in the German Army during the Great War of 1914-1918 -- a conflict subsequently known as World War I. Germany's defeat and subsequent humiliation in the post-war negotiations at Versaille, France, became a source of great bitterness for many Germans, not least the former army corporal and wannabe artist who would lead the National Socialist movement known as Nazism. The Nazi Party was not initially successful, and Hitler would be sentenced to a brief period in jail for leading a failed revolt against the German government. While Hitler was in jail, however, he wrote the book that would become the bible of the fascist movement (a movement already well-entrenched in Italy under the rule of Benito Mussolini, Hitler's role model). Mein Kampf, or My Struggle, was part autobiography, part political creed, and all antisemitic bile. In it, Hitler laid the blame for the Great War, and for Germany's subsequent humiliation under the terms of the Versaille Agreement, on Europe's Jewish population. 

Hitler was obsessed with the notion of German, or Aryan racial supremacy, and he viewed Jews as not only responsible for the war, but for the moral decline of his beloved German nation. Note, for example, the following two passages from Mein Kampf, in which the author and future Fuhrer ruminates on the pernicious influence of "foreign" race defilers -- read: Jews -- on the German people:

"What history taught us about the policy followed by the House of Habsburg was corroborated by our own everyday experiences. In the north and in the south the poison of foreign races was eating into the body of our people, and even Vienna was steadily becoming more and more a non-German city."

. . .

"One ought to realize that for one, Goethe, Nature may bring into existence ten thousand such despoilers who act as the worst kind of germ-carriers in poisoning human souls. It was a terrible thought, and yet it could not be avoided, that the greater number of the Jews seemed specially destined by Nature to play this shameful part. . . I began then to investigate carefully the names of all the fabricators of these unclean products in public cultural life. The result of that inquiry was still more disfavourable to the attitude which I had hitherto held in regard to the Jews."

Jews, in short, were responsible for all of Germany's ills, and, what's more, provided the intellectual underpinnings and revolutionary zeal behind the Bolshevik movement that had seized power in Russia towards the end of 1917. The key for Germany's revitalization, then, was in casting off the highly discriminatory conditions of the Versaille Agreement, which required that Germany claim responsibility for the Great War despite the obvious role of other great powers in its initiation; that Germany pay financial reparations to the victorious countries; and that Germany cede important territory to those victorious countries. The Great Depression also, and significantly, contributed to the environment in which Hitler's hatred towards Jews and other "race defilers" could take root among the German (and Austrian) people and lead to systemic oppression of the Jewish populations of Europe. Difficult economic times breed resentment among most peoples, and they are often all-too-willing to point their collective finger at whomever they feel can bear the brunt of their rage. Hitler's success in leading Germany's population to point that finger at Jews led directly to the establishment of a sizable infrastructure of concentration and extermination camps designed for the sole purpose of ridding the world of Jews, as well as homosexuals, Roma (Gypies), and others. 

The roots of the Holocaust, in short, lied in the hatred of Jews that has existed throughout much of history. Hitler was successful in exploiting German fears of nefarious outside forces because the German people were anxious for a leader who would restore their nation's greatness. Jews were an easy and attractive category of people to blame for their problems, and the overwhelming majority of Germans, Austrians, Poles and others were more than willing to cooperate in the extermination of the continent's Jewish population. That Jews were, obviously, Jewish and not Christian made them logical targets for Nazi Party castigation. Hitler's success in blaming Jews for past wars and for Europe's present difficulties could only occur because millions of Europeans were willing to agree that this one particular people were responsible for the economic difficulties that had swept the globe with the Great Depression and because they continued to blame Jews for the death of Jesus. Hitler instilled in Germans and Austrians the belief that Jews were an impediment to the racial purity necessary to reemerge as one of the world's great powers.