Triggering the Yellowstone Caldera, which hasn't erupted in 640,000 years, would spread ash across the country. Above, a map from the United States Geological Survey predicts the amount of ash from the blast
Conventionally, the non-assurance of “mutual assured destruction” was the greatest threat to its success. (E.g., the prevalence of such sentiments as, “Don’t worry, they won’t actually annihilate us‼ ”)
Maybe the 10000° heat from the explosion would create a permanent glass cork on top of yellowstone, keeping that volcano in check for another 500000 years?
Putin: "Hey! America! Happy to help with your little super volcano problem! You are welcome."