There is arguably no stronger vindication of realist thinking in international politics than when two states that are bitter ideological adversaries join forces to counter a third state that threatens both. Such alliances of convenience have ranked among the most consequential geopolitical events of the last century, memorialized by the now (in)famous photographs of Franklin Roosevelt (and Winston Churchill) sitting alongside Joseph Stalin at Yalta during the closing months of World War II and of Donald Rumsfeld enthusiastically shaking hands with Saddam Hussein at the height of the Iran-Iraq War. The springboard for Mark Haas’s excellent book Frenemies is that realist theories are glaringly incapable of explaining the several less memorable, but no less consequential, instances in which states failed to create such frenemy alliances, which he defines as “security cooperation between ideological enemies when those rivals confront a common and pressing security threat” (p. 13). Haas valuably identifies the ideological factors that facilitate or inhibit the formation of these alliances, enabling policy makers to better ascertain their prospects in future geopolitical confrontations.

Haas argues that two variables determine whether a frenemy alliance will emerge among endangered states. The first is regime vulnerability: whether an endangered state’s regime (or its dominant ideology) is vulnerable to being subverted domestically by a rival ideology. This variable establishes how costly a frenemy alliance will likely be to the regime. If it is highly vulnerable to ideological subversion, the domestic costs of allying with an ideological enemy will be high, and it will consequently be disinclined to form a frenemy alliance. Conversely, if it is minimally vulnerable to ideological subversion, the costs of allying with an ideological enemy will be low, and it will be amenable to a frenemy alliance.

The second independent variable is the configuration of ideological distance between a state and both its potential frenemy and the militarily threatening third party. This variable establishes the degree of necessity associated with the prospective alliance. The degree of necessity and a state’s willingness to ally with an ideological enemy will be low in three scenarios: (1) the state and the shared material threat possess the same ideology, which is different from that of the prospective frenemy (Haas dubs this scenario “ideological betrayal”); (2) the prospective frenemy represents the state’s foremost ideological danger, and the shared material threat is its foremost material danger (“divided threats”); and (3) the state, the potential frenemy, and the material threat all exhibit different ideologies (“ideological equidistance”). By contrast, the degree of necessity and a state’s willingness to ally with an ideological enemy will be high in two other scenarios: (4) the state’s most pressing ideological threat also happens to be its foremost material threat (“double threat”) and (5) the state’s prospective frenemy and the shared material threat possess the same ideology, which differs from that of the state (“ideological outsider”). Consequently, a state is most likely to form a frenemy alliance if the domestic costs are low and the necessity is high, and it is least likely to do so if the costs are high and the necessity low.

Haas tests the theory in three impressively researched empirical cases, drawn from the interwar, Cold War, and post–Cold War periods. During the 1930s, Britain and France failed to ally with the USSR despite the escalating material threat posed to all three states by Hitler’s Germany. This is because, for most of this period, conservative or moderate governments that were very fearful of the domestic and international spread of communism held power in London and Paris. During the 1970s, Chinese foreign policy shifted dramatically from spurning anti-Soviet security cooperation with the United States during communist hardliner Mao Zedong’s final years in power (1972–76) to actively seeking it under his successor Deng Xiaoping, who pursued market reforms that narrowed the Sino-US ideological gap (1979). During the late aughts, the Islamist and increasingly illiberal AKP Party that had governed Turkey since 2002 abrogated anti-Iran security cooperation with Israel because of its weakening grip on political power and ideological alienation from liberal democratic Israel. Notably, at the end of each case, Haas carefully rules out alternative realist explanations for the absence, delay, or attenuation of the frenemy alliance, such as the potential frenemies’ desire to buckpass or avoid intensifying the security dilemma against the shared material threat.

Haas’s book exemplifies how qualitative research in IR can be theoretically innovative, methodologically rigorous, and empirically exhaustive, while at the same time being highly accessible and policy relevant. The most important policy implication of Haas’s theory is that US policy makers should not overestimate the likelihood that illiberal Asian states that are materially threatened by China’s rise—a lengthy roster that includes Russia, Indonesia, and Vietnam, as well as the US treaty allies Thailand and the Philippines—would be willing to balance Chinese power alongside the United States.

Although Haas’s theory and findings are persuasive, they nevertheless leave some lingering questions in their wake. First, why was the Nixon administration so eager to ally with China in the early 1970s in the face of Mao’s stubborn reluctance to reciprocate? From the perspective of Haas’s theory, it is puzzling that Nixon, a rabidly anticommunist conservative Republican, courted Mao far more aggressively than his two Democratic predecessors, John F. Kennedy and Lyndon Johnson. Although the gap in ideology between all three liberal (small-d) democratic presidents and Mao’s regime was wide, it was slightly narrower between Nixon’s two more left-leaning (big-D) Democratic predecessors and the communist Chinese leader. This means that Kennedy and Johnson should not have been less inclined to ally with China than Nixon. The conventional wisdom among foreign policy analysts is that “only a Nixon could go to China” or, in other words, that only a vociferously anticommunist Republican could strike a rapprochement with the Chinese communists without being eviscerated by right-wing domestic opponents. Haas’s theory does not easily accommodate the possibility that an ideological hardliner may be more likely to possess the domestic political capital to ally with a state exhibiting an enemy ideology than a moderate who is closer ideologically to the prospective frenemy.

Second, how should scholars code the ideology of the United States under Donald Trump? At this point, it is hard to dispute that Trump was sui generis among US presidents because of his illiberal and antidemocratic beliefs, as epitomized by his incitement of the January 2021 insurrection on Capitol Hill. In Haas’s concluding chapter, he argues that Rodrigo Duterte’s 2016 election to the presidency of the Philippines precipitated a weakening of the US–Philippine alliance because Duterte’s illiberal left-wing populist ideology more closely resembled that of China, the shared material threat, than that of the liberal democratic United States. This interpretation of Philippine policy becomes more problematic if the United States under Trump is recoded as having exhibited an illiberal right-wing populist ideology that narrowed the United States’ ideological distance from Duterte’s Philippines.

Despite these and other quibbles, Frenemies stands out amidst an increasingly crowded IR literature on alliances because Haas’s ideational theory deftly fills a large explanatory gap left by the realist theories that have dominated that literature to date. It thereby merits a prominent place on the bookshelves and syllabi of international security scholars.