The Yellowstone caldera is the remains of a super-volcano that has erupted three times during the last two million years, the most recent 640,000 years ago. The caldera isn't dead; it could erupt again any time. In fact, lesser volcanic eruptions have occurred many times since then.
Such an eruption would be a world-significant event and would devastate life in almost all the western United States. Compare the previous eruptions' ash fall with that of Mount St. Helen's ash fall:
I found this little article on alleged contingency planning in the event that the Yellowstone caldera erupts. I don’t know how reliable the site is; today is the first time I’ve looked at it. Intriguing, at least for me. And much less depressing than reading most of the political commentary out there.
I’m reminded of sf writer H. Beam Piper’s Future History series, where much of what was left of western civilization is relocated to the southern hemisphere before a nuclear war devastates the northern hemisphere.
But if the Yellowstone caldera exploded at the same time as a North Korean EMP warhead a few hundred miles overhead, we'd really be screwed!