AT A recent conference for offshore wealth managers in Geneva, Basil Zirinis of Sullivan & Cromwell, a law firm, began his presentation with a discussion of events in Iraq, where Islamist fighters were advancing on Baghdad. Barack Obama, he claimed, was drawing a red line around the city and, if necessary, would "drop FATCA on them". Worse, they would get no deadline extension. The nuclear option, he added, was to treat them as if they were Swiss.

The analogy was tasteless, but also telling. FATCA stands for Foreign Account Tax Compliance Act, an American law passed in 2010 to crack down on the use of offshore banks, particularly in Zurich and Geneva, to hide taxable assets. The law, part of which takes effect on July 1st, is the most important and controversial development in decades in the international fight against tax evasion.