When lawful permanent residents in the United States apply for naturalization to U.S. citizenship, the naturalization process culminates with an oath ceremony wherein all new U.S. citizens swear the following oath of loyalty to the United States:
I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty, of whom or which I have heretofore been a subject or citizen; that I will support and defend the Constitution and laws of the United States of America against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I will bear arms on behalf of the United States when required by the law; that I will perform noncombatant service in the Armed Forces of the United States when required by the law; that I will perform work of national importance under civilian direction when required by the law; and that I take this obligation freely, without any mental reservation or purpose of evasion; so help me God. (Emphasis added.)
By swearing this oath, it would appear, foreign nationals renounce all foreign citizenships when becoming a naturalized U.S. citizen. However, the oath is not the final word on the matter. The U.S. government does not control whether foreign nationals retain foreign citizenships; the foreign governments do, and each government handles the issue differently.
The Canadian government, for example, does not recognize the renunciation, so Canadian citizens who become naturalized U.S. citizens become dual citizens with all the rights of citizens in both countries. They can present their Canadian passport when entering Canada and thereby receive all the rights associated with that status, and present their U.S. passport when entering the U.S. and similarly receive all the rights associated with that status.
The German government, however, takes the renunciation contained in the U.S. oath ceremony very seriously. According to the German Missions in the United States, “If you willingly apply for a foreign citizenship and obtain it, your German citizenship is automatically lost. If you obtain a foreign citizenship without an application for naturalization, you remain a German citizen.”
It is thus important for U.S. permanent residents to research the laws of other countries of which they are citizens to determine whether becoming a naturalized U.S. citizen would jeopardize those foreign citizenships.