Questions concerning whether someone is a citizen of one country or another are generally resolved by treaty, a compact formed between two or more nations with respect to matters pertaining to the public welfare pursuant to principles of INTERNATIONAL LAW. One person might qualify for dual nationality, that is, citizenship in more than one nation, if he or she can satisfy the citizenship requirements of different countries.
International law also recognizes a rule labeled the "law of the flag," which determines the citizenship of persons born on ships. The rule is responsive to the citizenship laws of different nations and to treaties that are rewritten to fulfill new political conditions.
A child born of U.S. parents on a vessel anywhere in the world is a U.S. citizen. A child born in U.S. waters on a foreign ship is a citizen of that foreign nation when his or her parents are citizens of that country. If his or her parents are from a different country, provisions of treaty or international law apply. A child born on the high seas on a foreign vessel of parents from that same country assumes that country's citizenship and not the citizenship of his or her destination.