The tax reform passed in Washington this week is a gauntlet thrown down to Europe’s big taxers. This column warned of that possibility in March, but the reality has turned out worse for Europeans than seemed likely nine months ago. Once President Trump signs it, the law will open a new battleground for competition with Europe’s highest-taxing governments. It also challenges Europe’s fundamental attitude toward taxation.

The reduction in tax rates is only part of it. The bill will cut the top federal corporate tax rate to 21%...