LONDON — The United States and the United Kingdom were once the leaders on the global stage, the diplomatic duo tackling great crises.

Through two world wars, from Yalta to East Berlin to the Balkans, the sight of American and British leaders together signaled that a predictable order governed the affairs of nations. Self-interested, no doubt, but dependable democracies, anchored in the rule of law, whose collective power and iron will were welcome when countries were starving, freedom came under fire or chaos claimed lives.