The four works reviewed in this essay examine the evolving character of bilateral US alliances with Japan and South Korea. As Japan and South Korea constructed highly developed economies, their relations with the United States became more equalized. Rather than supplicating clients, both have become essential partners in Washington's geopolitical calculus. All of these authors see potential threats emerging to these alliances as a result of America's recent turn to isolationism; however, structural linkages at the institutional level remain intact. The United States continues to supply security to meet evolving regional demands through a bilateral framework.

Terence Roehrig's Japan, South Korea, and the United States Nuclear Umbrella examines the geopolitical implications of extended deterrence in Northeast Asia. The narrative is aptly historicized but focuses mainly on contemporary developments. Roehrig argues that while it is highly unlikely that the United States will ever launch a nuclear strike to defend Japan or South Korea, extended nuclear deterrence acts as a strong political signal of America's commitment to its allies and prevents further nuclear proliferation. This gives the nuclear umbrella sufficient value to merit its continued extension.

During the Cold War, Washington effectively guaranteed Japan's and South Korea's security against potential communist bloc aggression by placing both allies under its strategic nuclear umbrella. These guarantees were ad hoc and informal at first. They became more overt declarations in the aftermath of America's withdrawal from Vietnam. The United States needed to reassure its Pacific allies that it would stand by established security commitments. In South Korea's case, Washington also used extended deterrence to squelch attempts by Seoul to develop an autonomous nuclear capability. However, increasingly direct statements of extended deterrence to Japanese and South Korean political elites did not translate into open admissions of a US nuclear presence to host nation societies. Given the political sensitivities, American policy makers deployed nuclear forces to the region through subterfuge. In South Korea, US officials neither confirmed nor denied the existence of tactical nuclear weapons, while in Japan, nuclear forces were concentrated on Okinawa. Okinawa's reversion to Japan in 1972 was made conditional on the potential reintroduction of nuclear weaponry to the island in the event of an international emergency. It is widely acknowledged that US forces carrying nuclear ordnance regularly transited Japan throughout the Cold War.

Northeast Asia was integrated into America's Pacific security architecture through bilateral alliances. This made strategic sense. Washington's two main Asian allies adhered to highly distinct geopolitical orientations during these decades. Seoul's primary concern remained immediate threats emanating from North Korea. Therefore, it welcomed the full spectrum of American military power coming to its defense. Tokyo, by contrast, was not deeply interested in the superpower disputes that typified the era. Japan focused on rapid economic growth and deferred its security responsibilities to the United States in exchange for allowing Washington to deploy its strategic forces across Japanese territory.

Roehrig convincingly demonstrates that seismic geopolitical shifts since the Cold War have radically altered the security calculus of all three players. North Korea's nuclearization and China's ascent to great power status have increased military threats to America's allies. Japan and South Korea repeatedly sought reassurances of US nuclear guarantees, thus complicating efforts to scale back America's global nuclear posture, particularly during the Barack Obama administration. Roehrig asserts that Washington has done much to allay its allies’ anxieties. Joint consultation mechanisms and observations of US strategic nuclear forces have become regular features of alliance relations. Yet, Japan and South Korea have not fully converged in their security concerns. Both see North Korea's growing nuclear arsenal as an immediate danger to their societies but view China's rise through different optics.

For Tokyo, China's growing economic power and military modernization have the potential to downgrade Japan into a second-rate power with little ability to influence regional geopolitical dynamics. As a result, Japan is quite willing to cooperate with the United States to contain Chinese ascension. The initial impetus to integrate Japan into an American-led ballistic missile defense system may well have been North Korean nuclearization, yet it is clear to all parties concerned that this defensive shield can also be utilized to counter China's offensive missile capabilities. Smoldering territorial disputes over the Senkaku Islands further increased demands for the forward deployment of American conventional forces in Japan after 2010.

South Korea, on the other hand, has pursued divergent policies toward North Korea and China. Pyongyang's expanding nuclear arsenal has generated substantial hostility in South Korea, particularly among conservative political elites. To be sure, ordinary South Koreans have displayed more than a modicum of security fatigue, viewing the North's antics merely as attempts to extract economic concessions through nuclear blackmail. Rightist governments are less sanguine. The Lee Myung-bak (2008–13) and Park Geun-hye (2013–17) administrations reacted to North Korean threats with economic retaliation and large-scale military exercises in conjunction with the United States. Post–Cold War relations with China followed an altogether different trajectory. Seoul normalized relations with Beijing in 1992, and by 2003, the People's Republic of China had become South Korea's largest trading partner. Moreover, Beijing has tremendous leverage to dissuade Pyongyang from excessive provocations. This makes South Korean elites highly reluctant to join US efforts to contain China.

However, North Korean nuclear diplomacy has made the American nuclear umbrella indispensable. It will remain so if North Korean threats persist and if the United States remains willing to extend it. President Donald Trump's rhetorical pronouncements to fundamentally revise alliance commitments to Japan and South Korea were popular with his electoral base, yet these sentiments did not seep into the bureaucratic departments responsible for maintaining defense cooperation. Institutional security linkages with these states have been maintained.

Extended nuclear deterrence does not mean probable use in Roehrig's estimation. Rather, the nuclear umbrella is a political signal by the United States that it is willing to stand by its allies with the full panoply of its military power. Such guarantees are not extended lightly. States covered by them can rightly consider themselves as ranking among America's top allies. Should strategic contingencies arise in Northeast Asia, it is far more likely that the United States will react by conventional means. Perhaps more importantly, the US nuclear umbrella convinces Japan and South Korea, both potential nuclear powers, to remain in a state of perpetual self-denial. Weaponizing nuclear power in either country will raise a host of uncomfortable questions and assuredly trigger deep resistance from civil society groups. Furthermore, if either one goes nuclear, the other will probably follow suit. American's provision of nuclear deterrence erects bulwarks against proliferation and its attendant problems.

In The Evolution of the South Korea–United States Alliance, Uk Heo and Roehrig chart the increasing complexity of bilateral ties between two closely intertwined states. The authors chart a transition from a patron-client relationship, in which Washington offered resources for compliance, into a more equal partnership in which both countries work jointly to achieve common interests on the Korean peninsula and beyond. Given that state-level contacts between the United States and South Korea emerged in a Cold War context, the alliance was initially focused on the containment of North Korea. American policy makers concentrated on security rather than democracy. Popular democratic movements might threaten stability and, in doing so, encourage North Korean adventurism, hence they did not receive meaningful support. When potential democratic openings occurred in 1960–61 and in 1979–80, the United States acquiesced to the reconstitution of authoritarian political systems led by military strongmen. People power did not serve US strategic priorities.

Heo and Roehrig's study would have profited from a deeper consideration of wider geopolitical configurations in the Asia-Pacific. South Korea was a garrison state that acted as a buffer between Japan and the communist bloc. It also provided crucial inputs of military labor to help prosecute America's war in South Vietnam. Such services could not be provided as easily by a liberal democracy subject to mass electorates. Unquestionably, ordinary South Koreans in the early Cold War period did feel a sense of obligation to the United States for saving them from a violent North Korean conquest; yet, at key junctures, Washington's policies diverged from the popular will.

Heo and Roehrig emphasize that all this changed in the late 1980s. The Ronald Reagan administration reversed its unqualified support for Chun Doo-hwan's regime in 1987 and threw its weight behind democratization. A waning Cold War, coupled with mass movements for people power across the globe, compelled Washington to reverse course. Heo and Roehrig contend that democratization deepened the alliance because both countries now shared common values. The alliance expanded into new fields of advanced technology, regional stability, climate change, and cybersecurity as South Korea constructed a highly developed economy with a globally connected citizenry. Additionally, South Korea channeled its growing wealth into military modernization and was thus able to assume most of the burden for its own defense. The United States returned peacetime operational control to the South Korean military in 1994, though Washington continues to organize joint military exercises with the South to sustain effective interoperability.

However, Heo and Roehrig are careful to point out that expanding political freedoms also made anti-American sentiments more vocal and relevant. Progressive South Korean politicians with different ideas regarding America's role on the peninsula increasingly assumed high public office. Alliance relations fluctuated between highs and lows, largely depending upon US–South Korean approaches to North Korea. Progressive presidents actively sought to reach out to North Korea, encouraging the piecemeal development of its fragile economy in the hopes of gradually moving toward unification. Republican American presidents viewed North Korea through a security prism. Sanctions, hostile rhetoric, and brinksmanship deployed to compel denuclearization only had the opposite effect. North Korean leaders reacted to US pressure by expanding their nuclear capabilities, thus creating greater tensions on the peninsula.

Heo and Roehrig formulate a theory that US–South Korean relations peak when Washington and Seoul take a common approach to the North. Democratic American presidents tend to concur with their South Korean counterparts’ policies, be they hostile or conciliatory, toward Pyongyang. Alliance relations decline when progressive South Korean chief executives interact with conservative American presidents committed to pressuring the North into denuclearization. Such was the strained relationship between Roh Moo-hyun and George W. Bush from 2003 to 2008. Heo and Roehrig presumably expected similar turbulence under Moon Jae-in and Donald Trump. They can, however, certainly be forgiven for failing to predict Trump's erratic interactions with North Korea. Maximum pressure was followed by promising summitry that ultimately yielded little substantive change. Trump's opening to Pyongyang, together with a temporary cessation of large-scale military exercises, garnered much support from Moon, but peaceful coexistence on the peninsula remains a long way off. Perhaps Washington's geopolitical disposition toward Pyongyang matters far more to Seoul than an American president's ideological proclivities.

South Korea's position regarding the burgeoning Sino-US rivalry is ever more complicated. Heo and Reohrig see new and expanding sectors for US–South Korean cooperation that will allow the alliance to endure. Both states share much common ground on democracy promotion, climate change, high-tech infrastructure, and cyberspace. Yet, China will probably remain South Korea's premier trading partner for the foreseeable future. If this is the case, it would be unwise for any US administration to compel Seoul to choose between China and America. Bilateral linkages must be widened and strengthened in areas of common interest. With these linkages maintained, South Korea will support the continued presence of US military forces on its territory. This, in turn, will enable Washington to shore up Japan's security and make it easier to contain China from Japanese territory.

Kenneth B. Pyle's Japan in the American Century provides a thorough analysis of two countries locked in what George Kennan called “an unnatural intimacy.” Japan's hierarchical, ethnically homogenous, and deeply communitarian polity fell under the sway of America's universalistic liberal hegemony after defeat in war. Certain concepts in this book have appeared in Pyle's previous work, particularly his notion of Japanese conservative pragmatism. Nevertheless, he makes numerous thought-provoking interventions. Pyle's most original arguments focus on the impact and legacy of Franklin D. Roosevelt's wartime policy of unconditional surrender. Roosevelt had to rouse the American people from a deep-seated isolationism to achieve total mobilization. He achieved this by calling for the unconditional surrender of the Axis powers. Germany and Japan would not just be pushed back to their prewar borders through a negotiated settlement. They would be entirely remade as states and societies under American tutelage. As Pyle contends, unconditional surrender bred unconditional resistance. Such resistance could only be overcome by unconditional violence. This built up a sociopolitical and military-industrial momentum that led to the atomic attacks in 1945. After crushing Japan completely, the United States imposed itself on a cohesive nation-state and tried to remake it in its own image. The Japanese greeted democratization with ambivalence, for it did not come from within.

According to Pyle, such an invasive seizure of Japan was unnecessary as US officials soon made numerous compromises with established elites and interests. This argument is problematic in certain respects. Pyle himself admits that it would have been much harder for the United States to mobilize so completely had unconditional surrender not been deeply embedded as a collective war aim. In addition, Roosevelt could not have known that geopolitical developments after his death would compel the United States to reverse course. Finally, Pyle's assertion that Germany deserved unconditional surrender because the Nazis proved unwilling to negotiate, while Japan did not because it was willing to discuss terms prior to the war's termination, is not fully convincing.

Despite its traumas, Japan made the best out its subjugation. Through skillful diplomacy, Yoshida Shigeru and his cohorts engineered open access to global markets and raw materials while closing Japan's own economy to foreign investments. The United States accepted this because Japan was the keystone in its Asian security architecture. US military bases in Japan enabled Washington to conduct its wars in Korea and Indochina, and later to contain Soviet forays into the Pacific. These activities occurred without substantive Japanese participation. Japan focused on high economic growth, paying minimal attention to international security concerns. American pressures to rearm were deflected by appealing to a sacrosanct, US-imposed peace constitution that only allowed for a defensive military. Japanese elites rejected collective security arrangements geared toward aggregating the military power of member states. This freed up resources and societal energies for production and trade. Japan became a merchant-industrial rather than a military-industrial state. There were challenges to this disposition from within the ruling Liberal Democratic Party's (LDP) own ranks, yet they were beaten back by mainstream conservatives committed to the Yoshida Doctrine.

Pyle's discussion of other US security arrangements across the Asia-Pacific is welcome but merits further analysis. Bilateral security treaties with Japan, South Korea, Taiwan, and the Philippines did indeed give Asia's security landscape a “patchwork” hub-and-spokes character. Yet, this defensive configuration displayed interconnected complementarities as well. Had South Korea and Taiwan not become garrison states defending the strategic approaches to Japan, Washington would have pressed Tokyo much harder to rearm. One of the reasons Japan never had to assume greater defense commitments was because South Korea and Taiwan, in effect, shouldered those burdens for them.

Japan's Cold War–era political economy worked so well, and created so many vested interests, that when the communist bloc unexpectedly collapsed between 1989 and 1991, political elites simply could not cope. As Japan fell into economic stagnation and as the changing geopolitical environment eroded its free security, change was slow in coming. Nevertheless, Pyle stresses that it did come. Yoshida school conservatives were gradually displaced by more right-wing LDP policy makers. North Korean belligerency and a rising China called for a definitive end to the postwar era. In addition, the United States had been pressuring Japan for greater contributions to the alliance since the early 1990s. It was not, however, until Koizumi Junichiro's government (2001–06) that Tokyo began consistently participating in Washington's campaigns to secure the international liberal order. Even then, Tokyo refused to commit combat troops to America's war on terror. Military units dispatched on United Nation's peacekeeping missions were also not permitted to use force.

More recently, perceptions that US power has declined and America's turn to isolationist populism under Trump made for a veritable national security renaissance during Abe Shinzo's second premiership (2012–20). Japan began to pursue its overseas interests in more autonomous fashion. Abe's government reconstituted conventional national security structures to plan and advise him on the best course of action. Pyle suggests that Japan has the potential, and increasingly the national will, to strike out on its own in the world. Nonetheless, Tokyo remains heavily dependent on Washington for its security. America's conventional and nuclear forces still provide a defensive shield behind which Japan can build up its own hard power. Interoperability with US forces will provide far greater conventional deterrence capabilities than Tokyo is capable of mustering on its own. Furthermore, Japan is not yet willing to work toward genuine reconciliation with its Asia neighbors. Rightist LDP officials harboring revisionist historical sentiments that call on former colonial peoples to display gratitude for, rather than indignation against, Japanese rule have severely antagonized South Korea. The more Tokyo and Seoul remain apart, the more reliant they will be on the United States to balance against regional threats.

Pyle's survey concludes on an interesting note. If the United States is to construct a viable balance of power to contain Chinese revisionism, it must scale back its commitment to spreading universalistic liberal values. America should allow distinct sociopolitical systems to operate within its alliance relationships, otherwise it can only obtain reluctant engagement at best. Now, more than ever, Washington must settle for interdependence without convergence. This thesis is fascinating but misses a key point. US security arrangements in East Asia have always been interdependent without being convergent. Divergent allied interests and abilities necessitated a composite strategic architecture from the outset of American dominance.

In Japan Rearmed, Sheila A. Smith contends that Japan refocused its national priorities on security questions through an incremental process. Given the military's depoliticization since 1945, these initiatives came from civilian LDP elites enacting various laws that slowly returned defense specialists to the policy-making process. Fundamental departures from established practices required radical alterations in the sociopolitical and strategic environment. Smith stresses that throughout the postwar period, right-wing LDP officials probed the limits of Japan's defense-only military policy without success. Kishi Nobusuke's ill-conceived attempt to ram through constitutional revisions that would allow Japan to fully rearm and pursue its national interests by conventional means ended in disaster. The Anpo crisis in 1960 cemented the Yoshida school's hold on government for two decades. Nakasone Yasuhiro pursued security revision with more subtlety in the mid-1980s, but this, too, did not undermine established practices.

Tokyo's botched handling of the Gulf Crisis in 1990–91 initiated a protracted legislative agenda by right-wing LDP governments geared toward enabling power projection beyond Japan's immediate boundaries. This may not have been possible without changing public perceptions of the military. Change came about due to a series of domestic crises. The Kobe earthquake and Aum Shinrikyo attacks in 1995, along with the Tohoku earthquake, tsunami, and nuclear meltdown in 2011, saw the defense forces performing essential tasks in government relief efforts. In addition, the military's role in organizing Japan's ballistic missile defense system has further increased public trust. For most Japanese, uniformed personnel were no longer seen as nefarious agents bent on plotting coups or launching aggressive foreign wars. Rather, the armed forces protected public safety and territorial integrity.

Smith contends that Tokyo's defense policies are largely reactive in character. Increasing threats from North Korea prompted Japan's integration into the US ballistic missile defense system. A rising China prompted the redistribution of military forces away from the old Cold War frontier on Hokkaido and toward the East China Sea. These developments came about with much US encouragement. Presidential administrations and defense officials in Washington found they had less need to prod their counterparts in Tokyo to ramp up hard power. Like Pyle, Smith argues that the US-Japan alliance has endured and become more conventional because both countries now share common rivals. For Tokyo, a nuclearized North Korea and rising China are far graver threats to Japan than the Soviet Union ever was. American power is still the main thing keeping these threats at bay.

Yet, in Smith's view, America's turn to right-wing populism has made Washington less reliable. US withdrawal from the Trans-Pacific Partnership complicated regional integration efforts. Aside from the general alarm among LDP elites, certain conservative voices saw Trump's isolationism as an opportunity to pursue more autonomous foreign and defense policies. LDP revisionists might at last be able to close the door on the postwar era and go their own way. Japan stood at a critical inflection point that could reshape Asia's geopolitical landscape. This argument is slightly overdrawn. Although few could have foreseen subsequent events that led to Trump's fall and Joe Biden's hard pivot to Asia, established interests on both sides of the Pacific sharply mitigated America's isolationist turn. Trump's priorities lay in revising trade, rather than security agreements. Threats to revise defense arrangements did not emanate from ideological convictions, they were primarily utilized to obtain economic concessions. Washington's national security bureaucracy and Abe's personal diplomacy convinced Trump to reaffirm US commitments to the Senkaku islands’ defense. In addition, Trump signed on to Abe's vision for a free and open Indo-Pacific in late 2017. Moreover, US–North Korean nuclear tensions early in Trump's presidency heightened military coordination efforts between Washington, Tokyo, and Seoul, displaying the benefits of force aggregation in Northeast Asia. Aggressive American attitudes toward North Korea only increased Japan's value as a strategic asset. Trump's unexpected opening to Pyongyang caused much confusion in Tokyo, but the Hanoi summit's failure to push North Korean denuclearization forward in 2019 prolonged the strategic status quo.

By all indications, Japan is working much harder than it once did to keep the United States committed to its defense. Tokyo has shouldering this increasing burden because it feels an alliance with a far-off American hegemon is the best way to push a near Chinese hegemon away. Nevertheless, Smith argues that if Washington ceases to be a reliable partner, Tokyo will conclude that the costs of the alliance outweigh its benefits. However, such an outcome is unlikely unless there is an overt alliance failure wherein the United States flatly refuses to defend Japan's vital interests. The Biden administration's strategic priorities, premised on containing China through close cooperation with regional partners, make such a possibility remote. Japan will be the primary strongpoint from which to counter Beijing. Defense planners in Washington are widening interoperability programs while Tokyo's defense procurements still rely heavily on American weapons systems. Despite recent political turbulence, the US-Japan alliance endures.

Demands for US protection remain high in Japan and South Korea because regional security threats did not dissipate after the Cold War. Indeed, geostrategic dangers have become more prevalent in some respects. As needs changed, Tokyo and Seoul attempted to tailor American involvement to fit new purposes. Washington has, by and large, encouraged, and matched, allied moves. Japan is a more accommodating partner than South Korea due to the latter's economic links to China. Yet, Washington and Seoul still share much common ground on which to maintain and expand ties. Notwithstanding populist rumblings, senior US officials remain committed to sustaining institutional linkages with Japan and South Korean for the foreseeable future. It remains unclear if isolationism can percolate the bureaucracy.