That was quite the shock. Wednesday’s ruling from the Court of International Trade saying Donald Trump had wrongly used the International Emergency Economic Powers Act to slap tariffs at will on trading partners seems to the layperson no more than common sense.

But given the extent to which courts generally defer to the executive on national security issues, it’s a dramatic and largely unexpected interruption to one of his most elemental policies.

This is of course very much not the end of it. The so-called section 232 tariffs on cars and steel are unaffected by the ruling. Trump will appeal this decision to the federal circuit court; beyond that he has a pliant Supreme Court waiting for him if need be; there are other obscure pieces of decades-old legislation he can dust off to resume his tariff campaign. But it does underline that his trade policy is not just ludicrous in substance but vulnerable to events, be they the courts or the financial markets, or in this case the two contriving to work in tandem. Equity futures duly shot higher as the decision was announced.