The Supreme Court of the United States has ruled that U.S. citizenship is a constitutional right that cannot be taken away from a citizen who does not intend to relinquish it. Therefore, such actions as naturalization in a foreign country, travel on a foreign passport, employment with a foreign government, and voting in a foreign election do not automatically jeopardize U.S. citizenship.
Which Passport to Use
U.S. law requires U.S. citizens, including those with citizenship of another country, to enter and depart the United States on U.S. passports. Dual nationals may be required by the other country of which they are citizens to enter and leave that country using its passport, but they do not endanger their U.S. citizenship by doing so.
Obligations to Your Other Country of Citizenship
Dual nationality has no effect on your rights or your responsibilities as a U.S. citizen, or on your obligations to the United States (i.e., payment of U.S. taxes if required to do so; registration with Selective Service). The dual national may also have obligations to his or her other country of citizenship. Failure to fulfill any such obligations may have little adverse effect on the dual national as long as he or she is in the United States. However, if the dual national travels to his or her other country of citizenship, he or she may be forced to comply with those obligations. In addition, if the dual national encounters difficulties in his or her other country of citizenship, the ability of the U.S. Embassy to assist the person may be very limited since the other country may not recognize the dual national's claim to U.S. citizenship.
Dual Nationality in Ukraine
Ukraine does not recognize dual nationality. Local authorities will treat dual nationals entering the country on Ukrainian passports as citizens of Ukraine. This may include fulfilling requirements for mandatory military service. In addition, individuals who do not relinquish their Ukrainian citizenship upon acquiring U.S. citizenship face potential fines.
For additional information on dual nationality, please visit the State Department website.