Why is Taiwan Important for US Security

If Taiwan gets invaded by China, the China immediately threatens the security of the US west coast, large cities like Los Angeles, San Francisco, and Seattle. Strategically the US has always maintained defensive capabilities of Taiwan. According to Taiwan Relations Act, the US has the obligation to provide weaponry to Taiwan.
Taiwan is like a unsinkable aircraft career. The super high way can act as aircraft landing strip throughout the 400 kilometer long island. Taiwan is the powerhouse for the US technology of semiconductor industry. And Taiwan provides the vibrant possibilities of democracy for Asia. In other words, if Taiwan is invaded or infiltrated by Communist China, then US is doomed to become shaky in terms of economy, politics, and national security.