It’s the long-range ambition of the People’s Republic of China to create a new world order with China at its head. Xi Jinping, since 2012 the general secretary of the Chinese Communist Party, has reinvigorated the party’s commitment to its Marxist-Leninist foundations, resulting in an ideologically aggressive foreign policy, especially toward China’s neighbors. But Beijing’s strategy for a Sino-centric world order requires more than ideological exertion. China, which is now second only to the United States in defense spending, has a powerful military capable of winning a strategic regional war in defiance of American...

It’s the long-range ambition of the People’s Republic of China to create a new world order with China at its head. Xi Jinping, since 2012 the general secretary of the Chinese Communist Party, has reinvigorated the party’s commitment to its Marxist-Leninist foundations, resulting in an ideologically aggressive foreign policy, especially toward China’s neighbors. But Beijing’s strategy for a Sino-centric world order requires more than ideological exertion. China, which is now second only to the United States in defense spending, has a powerful military capable of winning a strategic regional war in defiance of American primacy. A victory against a U.S. ally would destroy our dominant geostrategic position in Asia.

In his rigorously argued and compelling book “The Strategy of Denial,” Elbridge Colby focuses on the military element of China’s ambitions. The book outlines how China, as it has grown wealthier and stronger, has also pursued regional military dominance. The U.S. stands in China’s way as the greatest power in Asia and the leader of an “anti-hegemonic” coalition whose strategy is to thwart China’s designs for regional dominance. China seeks to neutralize this coalition by attacking its most vulnerable members and demonstrating that U.S. defense commitments cannot protect them.

Given its crucial geographic position in maritime Asia and its special status as a “quasi-ally” of the U.S., Taiwan is China’s obvious first target. Mr. Colby makes a persuasive case that if the U.S. does not rapidly address its military shortfalls, China can successfully invade and occupy the neighboring island. An invasion is Beijing’s “best strategy”—better than a lower-intensity military campaign, which would likely fail. Mr. Colby marshals an impressive command of military history to explain why Beijing will need to occupy the island to achieve its goals and how such an invasion might be conducted. It is particularly significant that Mr. Colby, a former Department of Defense official, details how the U.S. and Taiwan could prevail in such a conflict while avoiding nuclear catastrophe.

Simply put, this book will define the basis for future debate about U.S. defense strategy in Asia. With it, Mr. Colby earns a place as an intellectual heir to the Cold War strategists who thought seriously about how to thwart Soviet designs. He also makes well-reasoned but bold claims about a potential war with China. For example, he argues that as long as U.S. conventional strikes against Chinese targets remain limited, Beijing would not risk nuclear escalation. Assuming that U.S. forces would defeat a Chinese invasion, Mr. Colby argues that Washington would then need to escalate the conflict to get China to accept defeat. Such escalation could include not only economic warfare against Chinese leaders but also a punishing American bombing campaign. Mr. Colby is courageous in forcing readers to think concretely about the unthinkable.

Despite the book’s many strengths, it relies too heavily on the assumptions of so-called political realism, a general theory of international political behavior that cannot fully explain the CCP’s obsession with Taiwan. The CCP defines the problem of Taiwan as a legacy of the Chinese civil war. In the late 1940s, the defeated Chinese Nationalist Party retreated to the island and tyrannized the much larger native population. This development frustrated the CCP’s plans to create a unified “One China” made up of lands once conquered by the Qing dynasty. When Mr. Xi rallies the cadre on Taiwan, his enemies are “separatists” (the ruling Democratic Progressive Party) working with “foreign hostile forces” (the U.S.) against “Taiwan compatriots” who supposedly want to rejoin this mythical unified motherland. The invasion threat grows as the desire for unification with China shrinks.

But “One China” is an imperial idea, not a geopolitical necessity. Beijing weaponizes the idea to justify its destruction of political, cultural and religious pluralism in Tibet, Hong Kong and Xinjiang. Next up is Taiwan, the last hold-out. None of these peoples physically threaten China, but all threaten CCP ideology. Geopolitics aside, only countries intoxicated by notions of reuniting a dismembered Volk will start world wars.

Democracy in Taiwan is China’s main problem: no democracy, no threat of “separatism.” Under authoritarian rule by Chinese nationalists, Taipei agreed that there was “One China,” the disagreement was over who ruled it. When, in the 1970s, Beijing and Washington inked a Cold War rapprochement, this fiction worked diplomatically for both the U.S. and China. But since then a new democratic nation of Taiwan was born that has no use for mythical origin stories of a unified fatherland. The change in regime on Taiwan from authoritarian to democratic changed geopolitics. With its focus on material power, realism could not have predicted such an outcome.

As the old diplomatic arrangement fell apart, China adopted a clear new Taiwan policy: regime change. It is isolating Taiwan and attempting to subvert its democracy, laying out terms for “peace” that only a quisling government would accept. The threat of invasion is critical to this policy’s success: If Taiwan does not cave under pressure, Beijing will remove Taipei’s duly constituted government by force. Washington must undermine the CCP’s dangerous imperialist ideology, bolster Taiwan’s democratic resilience, and firmly embed the island in a still-nascent free-and-open Asian order. The answer to Beijing’s regime-change strategy must be a U.S. policy of preserving the democratic regime on Taiwan. A more realistic approach to Taiwan will require such moves as placing more U.S. troops on the island to shore up Taiwan’s political will under pressure and work more closely with its military. In making his case that conquest is China’s best strategic option, Mr. Colby quotes Napoleon’s aphorism: “If you want to take Vienna, take Vienna.” The U.S. needs a new, related slogan: “If you want to defend Taiwan, defend Taiwan.”

The already taxing work of countering Beijing’s strategy has just become more difficult. The Afghan debacle weakened the U.S. in Chinese eyes and has heightened the terror threat in key “anti-hegemonic” countries such as India, Indonesia and Australia. The task of deterring Chinese aggression is urgent, and Mr. Colby’s book presents a needed path forward.

Mr. Blumenthal is director of Asian studies at the American Enterprise Institute and author of “The China Nightmare: The Grand Ambitions of a Decaying State.”