> surely you realize that Hiroshima and Nagasaki are large cities today?

I don't think you understand the massive difference in scale between detonating a couple of atom bombs vs. thousands of thermonuclear devices, each with at least an order of magnitude (~16 kt vs hundreds) more destructive power. Nevermind the vast fallout dispersal that would blanket the northern hemisphere at least, as well as the ridiculous amounts of soot in the atmosphere from the resulting firestorms that would, to put it mildly, be a bit of a setback for agricultural yields for a damn long time. You might be okay in those unaligned places, sure, bit if you're in roughly half of the world you're pretty much effed.