Very sorry to see your *best answer* selection distorts history to somehow imply that Roosevelt and the US were more responsible for WWII than they actually were. I am sure that Hitler would have preferred not to have to fight a war, if Poland, Paris, and Eastern USSR would simply allow him to occupy their territory without firing a shot. However, Hitler was more than happy to use his military to take what he wanted by force, including planning to invade and defeat the UK. No excuses can be made for Hitler--he was the primary cause of WWII when considering all the other world leaders combined. If allies had not declared war on Hitler, he would have still sought to conquer Paris (if nothing else to erase the embarrassment of the ToV).
Chamberlain was PM when Poland invaded and while he believed in trying to solve conflict diplomatically rather than with arms, HE was the one that declared war on Germany out of the anger and embarrassment of being played by Hitler. Chamberlain and the UK were not ready for war, however and it took 8 months and a change in PMs to Churchill to build up to war.
A conflict between the USSR and Germany was likely inevitable in any case, Hitler's attack was partly preemptive, but he also coveted the vast Ukrainian wheat fields to grow his master race, and Soviet oil fields to help power it. And if he had delayed, there is a good chand that the USSR would have eventually attacked Germany (probably 6-12 months later).
Yes, the Holocaust was inevitable due to Hitler and Nazism as it was an internal political tool, not an external diplomatic one. The Holocaust (or at least its roots) effectively began well before Hitler attacked Poland. The Jews were the persecuted scapegoats by the mid-1930s after Hitler became Chancellor, and after the Kristallnacht in 1938 more than 30,000 Jewish men were arrested and incarcerated in concentration camps--just the beginning.