Two weeks have passed since the meeting between Donald Trump and Vladimir Putin, but there is still no public account of what the two leaders said—other than their own self-congratulatory remarks. But the recent actions of the U.S. and Russian presidents suggest they may have discussed the role and ambitions of the North Atlantic Treaty Organization, taking steps toward a rebalancing of power that should worry Europeans and Americans alike.

President Trump provided one clue in his statements about Montenegro, a tiny Eastern...