How many poison pills does it take to kill a trade deal? Three, according to Donald Trump. Mexico and Canada are bending over backwards to preserve the North American Free Trade Agreement. But their tolerance for Mr Trump’s demands is wearing thin. It seems a matter of time before he declares America’s exit from “the worst trade deal ever”. The temptation to withdraw from the World Trade Organisation will grow as Mr Trump’s term wears on. Anyone who thinks he has dropped his vow to rip up the global trading system has not been paying attention.

Reporters used to joke that “Worthwhile Canadian trade initiative” was the dullest headline ever written. Mr Trump has made it exciting. A year ago it would have been hard to imagine Canada and Mexico teaming up against the US. Mr Trump has driven them together. One such poison pill is the demand that Nafta “sunset” every five years, which is like putting a recurring expiry date in a marriage agreement. Divorce becomes inevitable.

Another is the demand that half of all duty-free content for cars made in North America be sourced to the US. This would shred the regional supply chains that automakers have set up. A third is to scrap Nafta’s disputes resolution system, which gives investors protection against arbitrary contract-breaking. Mr Trump built his business by dishonouring contracts. He is not about to stop now.