In the holocaust Nazis sent Jewish people to gas chambers. They sent people like send sheep to the slaughter. Evidence from this horrific acts were collected by the Russians and the Americans. Denying such monstrosity is the first phase of trying to create such horror again. The first measure to avoid this horrible crimes is not to forget.
War crimes is when crimes against humanity are taking shelter under the bombs of war. Hamas by raping, killing and mutilating women and children, and by preventing Gaza people to run away from war, are doing horrible war crimes. One of which is using hospitals as human shelter and bases for terror attacks.