President Trump shocked markets and challenged the Republican consensus last week by announcing new tariffs on imported steel and aluminum. Now the world is wondering how much further he will go. Should corporate executives and investors expect a few high-profile but largely symbolic measures, or should they brace for more trade shocks?

The stakes are immense, but the signals from the White House are, as usual, mixed. Mr. Trump tweeted Friday that, “trade wars are good, and easy to win.” But on the other hand, he said in a...