A person may be a citizen of more than one country. This can happen automatically, based on the citizenship laws of the countries involved. It can happen by marriage. It can also happen by voluntary application based on a country's laws and requirements. For information about U.S. law and the potential issues regarding application for citizenship with another country, visit the U.S. State Department Services Dual Nationality web page.
Often when applying for citizenship with another country, individuals are required to provide documentation of lineage in the form of their own birth and marriage records and the birth, death and marriage records of their parents, grandparents, great-grandparents and beyond.