Bad policies sometimes lead to interesting economics. For example, the disastrous turn toward austerity after 2009 was a kind of natural experiment that led to an upwelling of good work on the effects of fiscal policy.

And I’m starting to think that the Trumpian trade war, which finally seems to have arrived, will lead to some good work on international economic policy. Like the work on fiscal policy, it will of course be ignored by people in power. But still worth doing.

In this little essay I’m going to offer some analysis and a few numbers based on a chain of thought that begins with Trump’s evident disdain for Japan and ends up in Atlanta, New Orleans, and Buffalo. What? Bear with me if you have a taste for things wonkish.

My starting point is a peculiar aspect of Trump’s climb-down on steel tariffs. The ostensible justification for these tariffs was national security; but as was widely noted from the start – although this probably came as news to the Trumpies – most U.S. steel imports come from American allies. And now Trump has granted exemptions to allies that largely vitiate the supposed point of the exercise.