On the front line of the superpower struggle between the United States and China, Taiwan has fashioned a defensive masterstroke. It has become indispensable to both sides.

In dominating the fabrication of the most advanced semiconductors, the giant Taiwan Semiconductor Manufacturing Company Ltd. (TSMC) has captured a technology that's crucial to the cutting-edge digital devices and weapons of today and tomorrow. TSMC accounts for more than 90% of global output of these chips, according to industry estimates.

Both superpowers now find themselves deeply dependent on the small island at the center of their increasingly tense rivalry.