In an interview in the summer of 1965, McGeorge Bundy, who as Lyndon Johnson’s national security adviser was an architect of the major U.S. escalation in Vietnam then just getting under way, was asked how the actual conduct of American diplomatic affairs differed from his perception of it when he was a dean at Harvard. Bundy replied that the first thing that stood out was “the powerful place of domestic politics in the formulation of foreign policies.”

It’s not a statement that should surprise, and yet it does. It surprises because presidents and their senior aides so seldom make this admission on the record. Sometimes they’re reluctant to admit even to themselves that their decisions in foreign affairs could be affected by cynical partisan maneuvering, by legislative agendas, by election-year imperatives, by careerism. Instead they rush to proclaim fidelity to that favorite adage of politicians first articulated by Daniel Webster during the War of 1812: “Even our party divisions, acrimonious as they are, cease at the water’s edge.”