Holocaust literally means "total destruction", Merriam-Webster says it is a "a mass slaughter of people; especially : genocide" in our context. The Armenian Genocide may then be considered a holocaust, though we tend to think of the Jewish Holocaust of the 1930's/1940's when we say the word. Literally, the Armenian Genocide is "a holocaust" but not "the Holocaust". Depending on your definition of "large", Hiroshima and Nagasaki may be considered holocausts, as may any other slaughter or obliteration with fire one can find in human history.
Antisemitism existed in Europe long before the Second World War, and existed in places other than what would become Germany. For example, the nineteenth century saw Jews expelled to the westernmost regions of the Russian Empire, along with pogroms against them. They were completely expelled from Spain following the completion of the Reconquista if they refused conversion to the Catholic faith. Even during the Crusades, the religious fervor fueled the slaughter of thousands of Jews across the Holy Roman Empire and France, and had been denounced as "Christ-killers" beforehand. Simply put, antisemitism wasn't new at the turn of the twentieth century.
Now, WWI deserves recognition because of the sheer change it ushered into Europe. The French Revolution was a wound to the Old Regime in Europe, but WWI sealed its fate. It was the fall of empires, with new governments being set up in their steads, a redrawing of the maps of Europe. I suppose it also deserves to be noted that this was the final war where cavalry played any major role, as the Polish cavalry during WWII was unable to help protect that nation from occupation. It's always a wonder to look at the diplomatic web that initiated the Great War, though.
With regards to the origins of WWII, I would go back to 1871 at the very least, if not further. The loss of Alsace-Lorraine and the humiliating defeat suffered by the French would have been fresh in the minds of soldiers on the Western Front, with reclamation of that region as a goal in the peace terms following WWI. We could go to say that the causes of WWI came about with the recession of the Ottoman Empire, and the Austrian Empire's (Austria-Hungary 1867-1918) acquisition of portions of the volatile Balkans. It's up to debate, really.