> While I agree at some point China is likely to be just as strong, militarily, as the US, I don't see a compelling US interest in using warships to defend, say, Japan's claim to groups of uninhabited islands in the middle of the South China Sea.
I think we both know that we are re-enacting the classic debate between isolationists and those who want to be more engaged. I think your questions are important - the U.S. shouldn't follow these policies out of inertia - but I think there are very good answers:
* Japan and S. Korea can't protect themselves against China. They can today, but not in 10-20 years, and geopolitics moves very slowly: What happens now determines the outcomes in 10 years and 100 years. As one example, what happened way back in the 1940s and 50s determines those future outcomes: That's why the U.S. now has bases in S. Korea, Japan, and all the Pacific islands. 19th century events may figure even larger: The U.S. became a Pacific power, and China's encounters with the West resulted in a semi-independent city in China's south (Hong Kong), in much of China's current distrust of the West, in the rise of Communism in China, allowed Japan and Korea to become major independent powers (instead of weak tributary states to China), resulted in Russia having more southern Pacific ports, etc. etc.
* It's widely believed that if the U.S. doesn't guarantee security in the western Pacific, chaos and war will break out. The nations there don't trust each other at all, but they (sufficiently) trust the U.S. and they trust U.S. power to maintain order. If they become concerned they are trapped in an anarchic environment with enemies, with their existence at stake and no means of restoring order, they will arm up and attack before they are attacked, etc. Not only would that cost millions upon millions of lives and trillions of dollars, but in the heart of the world's biggest and fastest-growing economic region it would create a world-wide economic calamity.
* Finally, it costs much less to influence events abroad than to live in a Chinese-dominated world. If China belligerence isn't stopped there, it could easily spread further, and then rolling it back would be almost impossible. In addition, Americans and the West are accustomed to living in a world where they write the rules and establish the norms - so accustomed, many Americans don't even realize it. Living in a world led or dominated by a Communist dictatorship would be a very unhappy experience.
> To paraphrase Pournelle, I'm happy to be a friend of freedom everywhere, but a guarantor only of my own.
Personally, I don't like this statement at all. Everyone believes in freedom for themselves; IMHO, the word hardly means anything in that context. Freedom, to be meaningful, almost necessarily means freedom for others.
(And I'd add that if everyone else had that attitude then nobody would have freedom. The U.S. wouldn't be able to make it on its own; if it doesn't help others, then nobody will help it.)