The U.S. is the only country that has taxation of worldwide income for all of its citizens, no matter where they live and regardless of how long they have been overseas. In fact, the U.S. is one of the the only countries that imposes what is known as a “diaspora tax” on its citizens.
If you are a U.S. citizen or a resident alien, the rules for filing income, estate and gift tax returns, along with paying estimated tax, are generally the same, whether you are in the United States or abroad. With taxation of worldwide income, your U.S. and foreign income is subject to U.S. income tax, regardless of where you reside. It is a citizenship-based income tax.