The skies around Taiwan are thick with Chinese fighter jets and nuclear-capable bombers, with Beijing flying 150 sorties through the edge of the island’s air defense identification zone in early October alone. Whatever President Xi Jinping’s precise intention — to bully Taiwan and its allies, the United States included; to provoke them; or to inflame domestic nationalism — it is not benign.

As Mr. Xi’s crushing of Hong Kong’s free institutions shows, the “peaceful” reunification between his communist state and democratic Taiwan that he called for once again on Oct. 9 inherently threatens all 23 million people who live on the island. A hegemonic China would menace Japan, Australia and the Philippines, destabilizing the entire Indo-Pacific region. “There should be absolutely no illusions that the Taiwanese people will bow to pressure,” President Tsai Ing-wen responded, appropriately, on Oct 10. President Biden’s top foreign policy aides have also registered their disapproval.

The urgent question, for Taipei and for Washington, is not how to verbalize opposition to Beijing’s ambitions, but how to back it up in terms of military deterrence. China has spent hundreds of billions of dollars acquiring the capability to attack across the Taiwan Strait; Taiwan’s defense minister recently warned that the People’s Liberation Army will be fully ready to mount an invasion by 2025. Countering this, by raising the potential costs to China, looms as the greatest strategic challenge facing the Biden administration and, probably, its successors.

Unfortunately, while China has been focused on Taiwan, the United States has been tending to wider geopolitical interests, especially in the Middle East. Taiwan itself has neglected its own defense budget, devoting just under 2 percent of GDP in recent years. Taipei has taken encouraging, if belated, steps to remedy that, including a supplemental bill that would spend $9 billion partly on anti-ship missile capabilities to fend off a Chinese fleet. The Biden administration has announced a geopolitical pivot to meet the China threat, which includes the recently announced sale of nuclear submarines to Australia; closer ties with Japan and India; and diplomatic backing for Taiwan such as an appeal, supported by the European Union, for the island’s inclusion in the World Health Organization.

Since Mr. Biden became president, the United States has sold $750 million worth of artillery to Taiwan and continued a deployment of Marine trainers to the island begun under President Donald Trump. The troubling truth, however, is that in recent years Pentagon war games and other assessments have shown that U.S. capabilities, even in combination with those of its allies, might not be sufficient to defeat a Chinese invasion. Even as it encourages more effort from Taiwan, Japan and others in the region, the United States itself needs to invest more heavily in the hard-power assets — especially naval forces — required to back up its commitments in East Asia. The president, however, proposed a defense budget that barely kept up with inflation, albeit with $5.5 billion earmarked for deterrence in the Pacific. On a bipartisan basis, the House has approved a bigger spending plan, with money for 13 new ships. That might impress China more than even the sternest words.