Slumbering for hundreds of thousand years, the Yellowstone Caldera decides to erupt in late December of 1941, just days after Pearl Harbour.
How will the eruption interfere with WWII? How will it influence the world at large, how will it influence the US in special?
Depends on how big and sustained it is.
A 200 Megaton explosion would wipe the plain states out and strike food from lendlease. Which means millions of Soviets die from starvation and malnutrition.
Multiple 200 Megaton burst would destroy the U.S., Northern Mexico, and Southern Canada drop world temperature .2 degrees causing harsher winter conditions.
A single 200 Gigaton Eruption would wipe out North America, plunge South America into an Ice Age, turn Australia into a Lush Jungle, render the Arctic Circle uninhabitable, shut down the fighting in Europe completely and shift focus to Africa and the Middle East.