When hundreds of lawmakers, diplomats and experts attend the annual Halifax International Security Forum this weekend, they will do so in a different—and far more dangerous—world from the one we lived in a year ago. The top challenge: China’s global ambitions and increasing belligerence toward Taiwan.

China is amassing military power at an alarming pace. It is increasingly obvious that the longstanding U.S. policy of “strategic ambiguity”—refusing to commit to helping Taiwan in the event of a Chinese military invasion—no longer...

Taiwan President Tsai Ing-wen oversees the commission of the first squadron of the upgraded F-16V fighters in Chiayi Air Force Base, Taiwan, Nov. 18.

Photo: TAIWAN PRESIDENTIAL OFFICE/VIA REUTERS

When hundreds of lawmakers, diplomats and experts attend the annual Halifax International Security Forum this weekend, they will do so in a different—and far more dangerous—world from the one we lived in a year ago. The top challenge: China’s global ambitions and increasing belligerence toward Taiwan.

China is amassing military power at an alarming pace. It is increasingly obvious that the longstanding U.S. policy of “strategic ambiguity”—refusing to commit to helping Taiwan in the event of a Chinese military invasion—no longer serves our national interests. China’s strident behavior evinces a level of seriousness and a narrowing time horizon for action. If there is any hope of deterring war with China and protecting our Taiwanese friends, it is imperative that the Biden administration and Congress make American strategy clear.

Many leading U.S. defense planners can see the Chinese moving on Taiwan in the next six years, a concept known as the “Davidson Window,” after former Indo-Pacific chief Adm. Phil Davidson, who warned Congress about such a timeline. But several stunning displays of force from the Chinese military this summer—testing hypersonic missiles, building nuclear silos and surging aircraft near Taiwanese airspace—have likely reduced that six-year horizon. China’s naval capabilities, the cornerstone of any invasion force, are significantly outpacing our own. China’s enhanced space capabilities could hinder our military in the runup to a Chinese operation.

There are also troubling domestic trends in China: Demographic and economic growth have stalled and may crash in the coming decades. China also lacks easy access to the resources needed to sustain long-term competition with the U.S. Capturing Taiwan’s economy, with its semiconductor industry, could be the ace in the hole China needs to revive its economic fortunes and claim permanent superpower status.

Xi Jinping also has ideological reasons to invade Taiwan. Mr. Xi’s toxic brand of hypernationalism demands that he enshrine his legacy by “reunifying” China through a hostile invasion, which he has vowed to do. Moreover, Taiwan’s flourishing as an independent nation is a powerful and public refutation to Mr. Xi’s claim that liberal democracy is incompatible with Chinese culture.

U.S. defense planners have every reason to take the Davidson Window seriously. Because of a series of poor policy decisions, America’s ability to project power across the world is under stress at the very moment China is reaching new strength. President Biden’s botched withdrawal from Afghanistan has led allies and adversaries to question the administration’s commitment to alliances. Most important, Mr. Biden has neglected to provide the resources needed to curb the Chinese threat.

After years of underinvestment, the U.S. Navy’s fleet stands at 296 warships, compared with China’s 355 ships. High demands, poor maintenance and unfocused political leadership have left our surface fleet in a dismal state of readiness. And unlike the Chinese Navy, American sailors have a global mission requiring constant forward presence around the globe.

Beyond the Navy, the Biden administration’s bias toward emerging technologies over conventional elements of deterrence threatens to hollow out the U.S. military further, as more platforms are retired than built. Without a major infusion of resources to fill the gap, this strategy could usher in a period of American weakness that would render the threat of a Chinese invasion of Taiwan more potent.

Strategic clarity would do much to reassure allies, many of whom are confused by Mr. Biden’s muddled rhetoric on Taiwan. In one press conference after another, senior diplomats have had to walk back or clarify the administration’s position on Taiwan because of the president’s own words.

Mr. Biden’s strongest words have been reassuring at times, but his actions haven’t demonstrated that he takes the Chinese threat seriously. Updating our Taiwan policy to match today’s reality would free our military and civilian experts to tackle the hard problems while sending Beijing the message it needs to hear. But such a policy change will be meaningful only if paired with a serious shipbuilding plan that demonstrates U.S. strength and resolve.

Many of these problems are reversible if we choose to act with the requisite urgency. Mr. Biden must publicly declare that an invasion of Taiwan—unlike previous U.S. embarrassments in Georgia, Syria and Ukraine—would be met with the full force of American military might.

Mr. Wicker, a Republican, is a U.S. senator from Mississippi and a member of the Armed Services Committee.

China’s hypersonic missile test demonstrates the next major war will utilize cyber attacks and unmanned vehicles striking from afar. So far the Biden administration is ignoring the warning signs. Images: EPA/Shutterstock/Getty Images Composite: Mark Kelly The Wall Street Journal Interactive Edition