Donald Trump and Kim Jong Un both received what they most wanted from their one-day summit in Singapore on Tuesday: Images of the two men shaking hands, talking across the table and getting along famously. Whether this photo-op summitry achieved anything beyond the bonhomie is a lot less clear.

In Mr. Trump’s telling, his willingness to engage in personal diplomacy has persuaded the young Kim to abandon the nuclear-weapons program that he and his forbears have spent decades building. Mr. Trump gave Kim the legitimacy of equal...