BEIJING—President Trump wants to slap an additional 10% tariff on imports from China over its role in the fentanyl trade, arguing that Beijing could help fix America’s drug crisis if only it stopped dragging its feet. If cutting off the production and export of chemicals used to make fentanyl could get Beijing in Trump’s good books and prevent a new trade war, then why wouldn’t China do it? The answer cuts to the heart of why the war against fentanyl has been so intractable, owing to a lack of trust between the U.S. and China and the whack-a-mole nature of...