[vtange]

Taiwan and Ukraine are very different.

The U.S. doesn't have much history promising Ukraine defense as it was part of the U.S.S.R. during the Cold War. In contrast, Taiwan was practically there since 1949.

Taiwan's economy is multiple times the size of Ukraine's and is comparable to the likes of Saudi Arabia. To not defend Taiwan would send a very bad signal to all American allies that unless you're worth more than Taiwan, we'll sell you out. Assuming China continues to grow more powerful, this also means South Korea would be in danger as well. This is not even accounting the fact that if China controls Taiwan, China will control a majority of sea lanes to Korea and Japan. If she wished then she could interfere with oil supplies to those countries.

Plus the U.S. already goes great lengths to restrict China from acquiring U.S. technology, much of which has supply lines and partners in Taiwan. Letting China take Taiwan would mean a massive change unless you think we'll just accept that every Taiwanese company now will be forced to put backdoors for the Chinese.