The US is one of a hand full of countries that require you to pay income tax on money earned abroad. If you are a US citizen that has moved to another country and received citizenship there, and you aren’t worried about having “the local government send you to the gulag”, then renouncing your US citizenship is probably the best financial decision you can make. Caveat: they make you pay up front when renouncing for all of the potential tax they would have earned from you had you stayed a citizen abroad for the rest of your life.