Well, Japan probably won't even know about the eruption untill after the attack, so the war is going to happen at the same time. Can anyone dig out wind directions etc at the time? Its going to make a lot of difference.
The Yellowstone supervolcano erupts every 600 000 years or so, and there is no fossil extinction pattern even among larger mammals, so its not going to be too serious for mankind as a species.
The USA is probably going to lose a lot of their food production, internal transports and industry regardless. Canada might also be wrecked.
Germanys declaration of war suddenly makes sense, and probably happens on schedule.
In Europe, without the USA, Hitler probably lasts for 12 months to two years longer. The big difference is going to be afterwards. Without the USA as a counterbalance to the Soviets, the Iron Curtain is probably going to go all the way to Belgium. France might escape, depending on whos got soldiers there. Norway -probably not.
Another possibility is that the Germans take out Hitler and his closest on their own, and sue for peace with the allies, counterbalancing the Soviets.
I don't know how the war with Japan would go, depends on wind direction and how long the eruption lasts. This is basically a version of the "WW2 without the USA" scenario, but with the US still absorbing a lot of Japans attention.