For many free-market advocates, the recent conservative dalliance with noncapitalist policies has been as stunning as it has been swift. While President Trump’s antipathy to free trade was well-known, his and other conservatives’ sudden embrace of aggressive antitrust actions was more surprising. Now self-described “national conservative” thinkers are pushing industrial policy—government planning that meddles with economic decision-making to promote favored industries or social outcomes.

This turn has been long in coming....