European finance ministers are worried. They say the United States' big tax reform bill contains measures that would unfairly disadvantage European business and contravene global fair-taxation rules. Are they right? Last week, the finance ministers of Europe's five biggest economies — Germany, France, the UK, Spain and Italy — wrote an anxious letter to their American colleague, US Treasury Secretary Stephen Mnuchin, and copied it to all senior Republican politicians in the Congress and Senate. The letter's thrust: The draft US tax bill, if passed as written a week ago, would represent a break with global fair-taxation rules as applied...