In the tiny Caribbean nation of Trinidad and Tobago, Kamla Persad-Bissessar, the leader of the opposition coalition in parliament, recently did something no other world leader has done: She read the U.S. Republican Party platform.

There she discovered that the GOP had called for repeal of the Foreign Account Tax Compliance Act, or Fatca, which is best understood as a license for IRS imperialism. The Treasury Department has used the law to demand that foreign countries change their own laws so their financial institutions report...