On Sunday, Nov. 9, 2014, a military plane of the US led coalition flies above the Syrian town of Kobani, seen from a hilltop outside Suruc, on the Turkey-Syria border. (AP Photo/Vadim Ghirda, File) Photo: Vadim Ghirda/AP Photo

A few weeks ago, Ted Cruz committed a shocking act of heresy against the Republican Party Establishment. “If you look at President Barack Obama and Hillary Clinton and, for that matter, some of the more aggressive Washington neocons,” he told Bloomberg News, “they have consistently misperceived the threat of radical Islamic terrorism and have advocated military adventurism that has had the effect of benefiting radical Islamic terrorists.” Cruz was cleverly making a point about the Obama administration’s intervention in Libya, which resulted in a failed state that has nurtured ISIS, but his attack cut much deeper than it might have first appeared. One of the supporters of that venture was Marco Rubio, Cruz’s primary rival for the affection of regular (non-Trump-loving) Republicans. Rather than frame his contrast with Rubio as a matter of personal judgment or partisan loyalty, though, Cruz defined his opponents in ideological terms (“the more aggressive Washington neocons”). Indirectly, he was reminding his audience of another country in the Middle East where neocon military adventurism has wound up benefiting Islamic extremism — and harking back to an older conservative approach.

While Trump has distracted the party with bombastic grossness, Cruz has undertaken a concerted attack on an unexpected weak point: the belief structure, inherited by Rubio, that undergirds the party’s foreign-policy orthodoxy, opening up a full-blown doctrinal schism on the right.

The Iraq War remains the Republican Party’s least favorite subject, but the principles that drove the Bush administration into Baghdad (without a plan for the occupation) have remained largely intact. Most Republican leaders still espouse the neo­conservative belief in confronting autocratic governments everywhere, that demonstrations of American military power will inevitably succeed, and that the championing of democratic values should inform all major foreign-policy strategy.

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.

The isolationist tradition has long been misunderstood to mean a policy that perished overnight on December 7, 1941, and that promoted complete withdrawal from world affairs. In fact, isolationist thought grew out of — and, in some ways, represented the apogee of — American exceptionalism.

It regarded other, lesser countries with disgust, a sentiment that bred the competing impulses to both be distant from the rest of the world and to strike out at it.

Isolationism dominated conservative thought from the end of World War I — as a reaction against Wilson’s costly democratization crusade, as Cruz implied — through Pearl Harbor. After the war, without losing its hold on large segments of the GOP, the worldview mutated in the face of communism. The Soviet threat intensified the contradiction between the desire to quarantine America from the communist contagion and to eradicate it. The old isolationists resolved the tension by developing a fixation on airpower as a substitute for diplomacy and land forces. American planes would allow it to dominate the world while remaining literally above it. (Airpower, wrote the historian Frances FitzGerald, “would allow America both to pursue its God-given mission abroad and to remain the virgin land, uncorrupted by the selfish interests of others or foreign doctrines.”)

Republican leaders opposed the Truman administration’s plans to rebuild Europe, create NATO, and station a huge land force in West Germany. Instead, they proposed a massive air force. The right’s belief in the efficacy of bombing was enabled by its indifference to widespread carnage among enemy civilians. Conservatives like Barry Goldwater proposed using nuclear weapons during the Vietnam War. “If we maintain our faith in God, love of freedom, and superior global airpower, the future looks good,” said Air Force general Curtis LeMay, who had also called for nuclear strikes against North Vietnam. (In 1968, LeMay ran as vice-president alongside the segregationist George Wallace, a campaign that prefigured Trump’s combination of populism, white racial backlash, and an ultranationalist foreign policy.)

Republican presidents like Eisenhower and Nixon, though, followed Truman’s internationalist program rather than the unworkable fever dreams of the right. The bipartisan embrace of internationalism sent isolationism into a long, slow decline, its ideas circulating but without influence, a philosophy for newsletter cranks. Eventually, the dominant Republican foreign policy evolved once more, into neoconservatism, which combined the Wilsonian fervor for exporting democracy abroad with the isolationist distrust of diplomacy. The neoconservative project imploded in Iraq, but still, even in the 2008 and 2012 campaigns, the lone voice of dissent on neoconservative foreign policy was the libertarian gadfly Ron Paul, who brought isolationism back into the conversation. The surprisingly durable support for an odd little man in poorly fitting suits who kept ranting about gold indicated a potentially underserved market for Republican discontent over Iraq.

The Paul version of isolationism, inherited by his floundering son, emphasizes the live-and-let-live principle. Cruz’s version is more bloodthirsty, putting him in touch with the current, freaked-out conservative mood while reviving the bombing obsession of the mid-century conservatives. “We will utterly destroy ISIS,” he boasted recently with LeMay-esque ghoulishness. “We will carpet-bomb them into oblivion. I don’t know if sand can glow in the dark, but we’re going to find out.” (Cruz’s choice of imagery is important: Conventional bombing does not make things glow, but nuclear bombing does.) At the debate, Rubio shot back, “Airstrikes are a key component of defeating them, but they must be defeated on the ground by a ground force.” When pressed by moderators on the details of their respective plans, both Cruz and Rubio retreated. Cruz admitted he would not, in fact, level the cities held by ISIS (which are populated mostly by their unwilling captives) but would instead simply bomb ISIS’s military positions, which Obama is already doing. Rubio admitted he would not dispatch an occupying force back to the Middle East but merely send a small number of special forces while attempting to recruit local Sunnis, which Obama is also already doing.

Substantively empty though their bluster may be, Rubio and Cruz are pantomiming a deep-rooted, significant breach. While he has very little support among party elites, Cruz seems to believe that Republican voters are hungry for a candidate who will challenge their party’s foreign policy at the ideological level. Very soon, we will find out if he is right.

*This article appears in the December 28, 2015 issue of New York Magazine.