North Korea has no nuclear program. Period end. Next issue please. Seriously. Iran's nuclear program poses a much greater threat to the United States and its interests. Trump wants North Korea because he foolishly believes he can amp up his dismal approval ratings and he believes the United States hasn't much to lose. That makes him wrong on two counts.
That's actually my point. Iran's nuclear program doesn't pose a serious threat to the United States proper, though it threatens our interests in the Middle East. North Korea poses even less of a threat.