From National Review:

The fundamental reason is, counterintuitively, China’s awesome power, and the very real danger that this power, if allowed to expand too far, will pose to Americans’ prosperity and freedom. The United States should defend Taiwan because it is important to deny China hegemony over Asia, by far the world’s largest market area. If China could dominate Asia, as it has made increasingly clear it seeks to do, Beijing would determine the terms, tempo, and distribution of global economic power. This would have the most profound and direct implications for Americans’ economic fortunes and, because our economic security is tightly linked to our freedom, it would ultimately endanger our liberties. A China dominant over Asia would have the power and wealth to dictate to Americans, fundamentally altering — and undermining — our national life.