Sometimes it’s worth thinking about why you think what you think. Take the Republic of China on Taiwan. Defending Taiwan is an article of faith in some quarters. Yet not everywhere.

A year-and-a-half ago an Asia security expert advising the American military and reportedly now under consideration for a job in the Trump administration told a small group, including me, that Taiwan should not be defended.  

The argument? The Chinese military is too strong, so we’d lose if we tried to save Taiwan. The American military would be savaged and our standing worldwide as well. Thus, we must let Taiwan go. Can’t be helped. That’s a handsome way to describe preemptive surrender.

America’s undersecretary of defense for policy, Elbridge Colby, now drafting our national defense strategy, doesn’t go quite as far, but in his confirmation hearing he remarked that Taiwan is important but not “existential” for America. So it’s worth reviewing why Taiwan’s freedom matters.

Let Taiwan fall under Chinese Communist control and several things happen — none of them good for America or the free world, not to mention the Taiwanese themselves. Militarily, it’s a huge advantage for the Communist China’s People’s Liberation Army.

For starters, the First Island Chain is broken. The PLA will have solved a longstanding strategic problem of how to penetrate the chain of islands stretching from Japan down to Malaysia that obstructs easy access to the Pacific. Properly defended, the islands hem in the PLA.  

Taiwan is right in the middle. Imagine a castle wall being breached. Operating from Taiwan, the PLA swings to the north and surrounds Japan. Move down to the south, and Australia is cut off.  

The PLA gets ready access to the Central Pacific and beyond.

This includes the hitherto “safe” zone formed by the Freely Associated States — Palau, Micronesia, and the Marshall Islands. This east-west “corridor of freedom” is the main logistics route for Yanks operating in the Western Pacific. Everything gets harder, or impossible, if the American military has to fight its way to the fight.

PLA objectives extend across the entire Pacific Ocean — all the way to Latin America. China is laying in the needed port and airfield infrastructure in South America. The new port at Chancay, Peru, is just one example.

Looking beyond the military aspect there’s a huge political and psychological effect from 23 million free people coming under Chinese Communist domination. 

First, it demonstrates that American military power could not prevent a free people’s enslavement. American financial and economic power and pressure against the PRC couldn’t either. As ominously, American nuclear weapons couldn’t stop the PRC.

Lose Taiwan and Asia will turn “red” overnight. Every country in the region will cut a deal with Beijing. The Japanese might resist but it will just be a question of how much freedom China allows Tokyo to pretend to have — and how much territory Japan can keep.

The Australians? Distance won’t save them. And Australia is a big place. So a million or two Chinese immigrants might be part of the deal too. Beyond Asia, every country anywhere that was counting on explicit or implicit promises of American protection would have serious doubts.  

The United States’s reputation would be shot. It would have trouble hanging on in the Asia-Pacific, and gradually it would be forced back to Hawaii and the American west coast. Globally it will be on the ropes.  

Even in the United States a constituency could be expected to declare that America must manage its decline gracefully. Taiwan is not the hill to die on, they’ll say. They never say which one is.

Mr. Colby may think Taiwan isn’t existential for America, but it is terrifying for Beijing. A vibrant Taiwan democracy puts the lie to Beijing’s claim that only it can govern China, since according to the Party, and Jackie Chan, Chinese people can only be ruled with a boot on the neck.  

How does the CCP explain Taiwan — consensually governed, with all the freedoms Americans take for granted, and economically prosperous? It can’t. And what about Taiwan and semiconductors?  It produces about 60 percent of the world’s chips and 90 percent of the highest-end ones.

Ultimately, it’s freedom that is at stake here. For just about everyone alive, America has always been freedom’s backstop. That was never guaranteed. John F. Kennedy and Ronald Reagan knew defending freedom was defending America. Taiwan is the bellwether.