An Eruption of the Yellowstone Caldera could been an extinction level event for the world, and would most certainly mean the end of the USA.
It's been some time, But I read once that the last Eruption at Yellow stone was so large the force actually changed the length of a day by slowing our spin, moved us slightly in space, and changed out Angle on our Axis. The Ash and rock it spewed into the Stratosphere sparked one of the deepest Ice Ages in Earth History.
Wherever you're getting your information from, I would stop if I were you. Volcanic activity from the Yellowstone hotspot continues to this very day, and is seen in the form of geysers and ground swelling. Aside from the handful of super-eruptions the field has also exhibited non explosive lava flows. The last super-eruption out of the Yellowstone hotspot spewed about 1,000 km/sq of ejecta, which is the bottom qualification of a "super-eruption." Most explosive eruptions produce more.
I think that whatever you were watching you are confusing Yellowstone with the Toba Caldera. The last super-eruption there was one of the most massive ever in history. The catastrophe theory behind it attributes the eruption to accelerating the onset of the last ice age, though not causing it by itself. I've never heard of a theory that it caused a change in the earth's orbit or rotation, though I suppose it's possible considering the multiplicity of factors that are involved in the position of the earth's axis (with tectonic activity being one of them). The eruption caused a volcanic winter, the duration of which is disputed among scientists, though certainly no more than a decade. Though some scientists theorize that any effects in weather may have endured for no longer than three years. The Toba eruption is attributed as being a leading cause in a bottle neck in the Homo Sapiens population, to possibly as few as 10,000 people, and with possibly being a major contributor to the extinctions of Homo Erectus and Neanderthals.
All this being said, it needs to be understood that this event was an exceptional case of volcanic activity. The chances of the next Yellowstone eruption even equaling the magnitude of the Toba eruption are negligible. The chances that a comparable eruption could be an "extinction level event" for humans are effectively zero, considering the differences in human population dispersement of then and now. When the Toba eruption occurred humans occupied mostly native regions in Asia and Africa, with African populations being the predominant survivors from which the population was replenished. Scientific evidence indicates that the human population quickly rebounded and increased exponentially after the sudden decrease. A Yellowstone eruption the size of the Toba eruption would indeed have a great effect on the US, and could certainly lead to a decline in population over the next couple of generations. But the more likely case is that modern technology and economics would largely shield the ability of such an event from inflicting substantial damage to the US population, while other regions around the world might suffer more from any food shortages, due to the US having greater economic ability to secure scarce food than poorer countries.
Currently, there is no evidence that the Yellowstone hotspot will erupt in any forseeable future. A few years ago there was a sudden increase in the growth of the Yellowstone plateau. However, that subsided and returned to normal levels.