With his announcement Thursday that he is imposing steel and aluminum tariffs not only against China but also on Mexico and Europe, President Trump is taking trade policy in exactly the wrong direction.

The president’s awful misunderstanding about trade is reflected at least partly in the fact that the Dow Jones industrial average, about which Trump so likes to brag, is down by nearly 500 points this week and by more than 2,000 points from its high this year. It’s no coincidence that this sharp slump covers the period since Trump’s first mention of steel tariffs.