What do they have to gain by destroying America? Unlike IS who seem to have no qualms about killing themselves for their cause because they believe they'll be rewarded in the afterlife, NK leaders are not suicidal. They aren't building a nuclear arsenal to obliterate the US & get wiped out in return by it or its allies, they're building it so they have some strength at a negotiating table. They don't have natural resources to export & aren't a country rich in anything anyone else wants. So they're building an arsenal they can use to threaten others & buy the support of allies. And presumably as an export. None of which we want to happen, but not worth starting WWIII over. The US fires first & they &/or China & Russia will hit the US.