Europe’s war on low taxes went on the offensive again this week, with a ruling from Brussels that not taxing enough now constitutes a form of illegal state subsidy. Really.

Competition Commissioner Margrethe Vestager reached that conclusion in a case concerning how much tax Starbucks pays to the Netherlands and Fiat Chrysler to Luxembourg. Each company based a subsidiary in the offending jurisdiction and then arranged intra-company transactions between those and other units in a way that the Commission says was designed to...