When he first came to Washington, Rubio distanced himself from these beliefs. “I don’t want to come across as some sort of saber-rattling person,” he said in 2012, the next year insisting that higher military spending be paid for with offsetting cuts elsewhere. The next year, he started rattling sabers. Rubio came to support higher defense spending even if it increased the deficit, and turned sharply against the Iran nuclear deal. Now a full-scale hawk poised to restore the banished Bush doctrine, Rubio has surrounded himself with neoconservative advisers, using buzzwords like “moral clarity,” and promised to stand up to Russia, China, Cuba, and North Korea, unworried by the possibility that standing up to some of the bad guys might require the cooperation of other bad guys. “I’m ready for Marco,” enthused William Kristol.

The Bush years trained liberals to think of neoconservatism as the paramount expression of right-wing foreign-policy extremism. But neoconservatism runs against the grain of an older and deeper conservative tradition of isolationism. Cruz has flitted about the edges of the libertarian right, sometimes forming alliances in the Senate with Rand Paul, an isolationist who — after briefly being in vogue — has largely been marginalized within his party. At the last Republican foreign-policy debate, Cruz identified himself with that creed more openly than he ever had. Just as Rubio’s buzzwords signal his neoconservative affiliation, Cruz conveyed his isolationism by calling for an “American-first foreign policy” and dismissing Rubio as a “Woodrow Wilson democracy promoter.” The face-off between Rubio and Cruz at that debate represented something far more profound than the usual exchange of canned sound bites.