As London dispatches Prime Minister Keir Starmer to Beijing with a business entourage, vowing a "clear-eyed" approach to national security, the symbolism is hard to miss. Europe is hedging: away from American tech, not necessarily toward Chinese tech—but toward strategic room to maneuver if Washington under President Donald Trump turns hostile.

Strategic ambiguity has certain domestic political advantages for Mr. Trump as well. China policy is one of the few areas where Republicans in Congress have found a tremulous voice to criticize the administration, reproaching its softer moves toward Beijing. Ambiguity keeps them hoping for a turn back to hard-line moves.

President Donald Trump on Saturday threatened to slap 100% tariffs on Canadian imports if America’s second-biggest trading partner makes a trade deal with China.