Five years into a once-unthinkable trade war with China, US Treasury Secretary Janet Yellen chose her words carefully on 20 April. In a wide-ranging speech, she reversed the terms of US engagement with China, prioritising national-security concerns over economic considerations. That formally ended a 40-year emphasis on economics and trade as the anchor to the world’s most important bilateral relationship. Yellen’s stance on security was almost confrontational: “We will not compromise on these concerns, even when they force trade-offs with our economic interests.”

Yellen’s view is ...