On earth of course, which has happened lots of times already, going back to Hiroshima in 1945.
Wrong. An attack on US territory would bring about MAD result, end of civilization. The only way we were able to use atomic weapons in WWII was because Japan didn’t have a bomb.
Space warfare is just emerging. No rules yet. The US relies on space for it superiority in warfare and that’s also a vulnerability. If GPS went away, who suffers most? The US and western world. Same with many other C4I capabilities.
Enemies are going to exploit this vulnerability. And the results will affect military and commercial systems alike. The radiation effects alone will take out most commercial system in hours or days. Space becomes a useless wasteland of Kessler breakups. Now if an enemy does explode a nuke in space, would that trigger MAD? Likely not. War yes, but a nuclear exchange, doubtful.
Like I asked, think about the risk of putting more critical infrastructure in the most fragile place possible.