Just when you think President Trump has reached a moment of trade equilibrium, there he goes again. Mount Tariff erupted once more Monday morning in tweets announcing tariffs on Brazilian and Argentine steel that dampened hope for election-year economic calm.

“Brazil and Argentina have been presiding over a massive devaluation of their currencies, which is not good for our farmers,” the President tweeted. “Therefore, effective immediately, I will restore the Tariffs on all Steel & Aluminum that is shipped into the U.S....