The question is both pertinent and impossible to answer easily, and it demonstrates why the study of the Holocaust remains important. If what is meant by the "start" of the Holocaust is the decision to exterminate the Jews , it can be argued that this was the inevitable conclusion of the logic of bigotry. That is, if the ills of society are blamed on the existence of a certain group of people – as the Nazis did with Jews – sooner or later someone will suggest that this inherently detrimental group should be eliminated. This almost happened in the United States with its shameful treatment of Native Americans, although they never crossed the line between bigotry and a decision to exterminate the entire group. The Nazis did.
One of the reasons that the Nazis could take the step of making their twisted vision a reality is that it was done gradually. The Nazis came to power in 1933 and the hatred of Jews was part of their platform and ideology. Indeed when the Nazis' mystical concept of the "Aryan" race is studied, it is clear that bigotry against non-"Aryans" was not only a necessity but was inseparable from their basic philosophy. Moreover Jews were an obvious presence in German society.
The first steps that the Nazis took against the Jews was to isolate them from the rest of German society. They were banned from schools and universities and prohibited from practicing certain professions and confined to living in certain areas. Again, it could be argued that these strictures were no less harsh than those applied to African Americans – or even Jews – in the US. But, again, the Nazis took this to the next level. In this case it was state-sponsored violence.
Violence against Jews was always a part of life in Nazi Germany. The S.A. – group of thugs known as the "brown shirts" – was one of the basic tools of the Nazi rise to power. When Hitler took power in Germany he did not reign in the brown shirts or attempt to modify their behavior. Instead they were allowed to run riot continuing their violence against the enemies of the Nazis such as socialists, union members, and, of course, Jews. Nothing was done to stop them until their leader, Ernst Roehm, became an actual danger to Hitler’s power. In the interim the violence against Jews grew from the random acts of thugs to events planned and sponsored by the government. The notable example is called Kristallnacht.
In 1938, after the assassination of a German diplomat in Paris, Nazi thugs, with the knowledge, approval and encouragement of the Nazi leadership, engaged in an orgy of violence against Jews and Jewish businesses. The reaction of the government was not to halt the violence but to imprison a large number of prominent Jews in Dachau. This violence occurred within Germany and Austria (which had been annexed to to Germany). The Nazis realized that if they could do this at home, they could do far worse in the distant regions of central Europe which, according to Nazi philosophy was the rightful property of "Aryans." From this start the violence against Jews grew even worse until it reached the horrors of the extermination camps.
If your wish to know when did the physical extermination of the Jews began you are asking a question with an equally difficult answer. Everyone knows that the last means used by the Nazis to murder Jews was the gas chambers at the Auschwitz concentration camps for that specific purpose. Few realize that Auschwitz was the end of the process rather than the beginning and that there is an active debate about the Nazis’ plans for the future.
The murder of the Jews started not at Auschwitz but with four small groups known as Einsatzgruppen sent with the German army in the invasion of the U.S.S.R. in 1941. Originally, in 1939, the Einsatzgruppen were proposed as a means to keep order in the conquered territories. Whether this was a serious proposal or not (and there are arguments on both sides) the purpose of the Einsatzgruppen was defined as the murder of Jews long before the actual invasion of the U.S.S.R. In fact the Einsatzgruppen did function exactly as intended. With the help of German soldiers and other Germans and local militia, the Einsatzgruppen shot hundreds of thousands of Jews in what is now the Ukraine, eastern Russia, and the Baltic states. They were especially effective in areas such as Lithuania where the Einsatzgruppen received the enthusiastic assistance of Christians in the area. (One of the last trials about the Holocaust occurred in Easton, Pennsylvania, less than ten years ago. It concerned a man named Stelmokas who was an officer in the Lithuanian army and cooperated with the Einsatzgruppen in the murder of the Jews of Lithuania.)
The Nazis had not anticipated, however, how slow this process would be, how messy it was or the effect on the men committing the murders. While it seemed efficient on paper, the pace of the exterminations was far slower than expected and destroyed the morale and the discipline of the men ordered to murder women and children. The final straw was that the actual scenes of murder were horrific. Himmler himself actually became sick when he witnessed a mass execution. The decision was made to find another method to exterminate the Jews.
The Nazis found the next step at home in Germany. One of the first steps towards the "racial purity" of the "Aryan race" was the elimination of "life not worth living." This included various experiments in the elimination of adults and children born with birth defects such as withered limbs and Down's Syndrome. These experiments included a small gas chamber at a facility in Hartheim, Germany. With the failure of the Einsatzgruppen the Nazi authorities turned to the expertise of the designers and operators of this gas chamber.
The first attempts the Nazis made to exterminate the Jews in gas chambers was at several concentration camps – notably Treblinka, Sobibor, and Madjanek– built for this purpose. While hundreds of thousands of Jews were murdered at these camps they were ultimately a failure. The murder camps were not as efficient as has been hoped as they required a large staff even when not in operation. After a revolt and escape by the prisoners at Sobibor the camps were destroyed and the active elimination of the Jews shifted to Auschwitz.
At the time that exterminations were shifted to Auschwitz, that concentration camp, located in the portion of Poland already annexed to Germany, had been in existence for several years as a series of concentration camps used mainly for slave labor. It was, in all probability, chosen because of its proximity to railroads running into the east. After some experiments, four gas chambers or "krema", were built at a sub-camp of Auschwitz known as Birkenau. These "krema" functioned to the end of World War II as the main means of the extermination of the Jews. The "krema" were destroyed shortly before Soviet troops liberated Auschwitz.
The last matter to be considered when thinking about when the Holocaust began is what the Nazis were really planning. It must be remembered that the plans of the Nazis were prematurely ended by their defeat. We now call the Holocaust the "Final Solution" and some even consider the genocide of the Jews to be unique. I believe that this is a mistake. The Nazis fervently believed that it was wrong that their superior race be confined to the narrow boundaries of Germany. They coveted the broad expanses of rich farmland to the east. In order to obtain these lands for the future expansion of the "Aryan" people they had to depopulate those lands. When they were defeated the Nazis were building new gas chambers. One was built at the concentration camp at Dachau although no one is sure whether it was used or not. It is my belief that, had the Nazis succeeded in their genocide of the Jews, the same fate awaited the Poles and the Slavs of eastern Europe. It is a chilling thought which makes the study of the Holocaust even more relevant.