None.
You're mistaking the Holocaust with World War II. In the strict definition the Holocaust refers to the killings of Jews in World War II.
But what I really wonder about:
how do you explain the massive amount of evidence? The video footage, the written documents, the pictures, the disappearing of millions of people, Nazis admitting what they did, ...?
I'd go as far as saying that if there is anything that historians can prove, it's the Holocaust.