The optimistic summit was a remarkable change in dynamics from less than a year ago, when Trump was threatening “fire and fury” against Kim, who in turn scorned the American president as a “mentally deranged U.S. dotard.” Beyond the impact on both leaders’ political fortunes, the summit could shape the fate of countless people — the citizens of impoverished North Korea, the tens of millions living in the shadow of the North’s nuclear threat, and millions more worldwide.