Chinese President Xi Jinping sent an unmistakably tough message on Taiwan to US President Donald Trump during their recent summit in Beijing. Xi warned that if the issue was handled poorly, China and the United States would “collide or even clash”, creating an “extremely dangerous situation”. He stressed that Taiwan holds the key to Sino-American relations.

This is a classic example of the bright red lines that Xi has grown fond of drawing in recent years. In 2022, on the sidelines of a G20 meeting in Bali, he warned US President Joe Biden of four red lines in US-China relations: Taiwan; democracy a...