Some people have dual citizenship because they were born in the United States to immigrant parents, or were born in another country to a U.S. citizen and a citizen of that country, according to FindLaw. Some immigrants who became naturalized U.S citizens maintain citizenship in their native countries.
The United States doesn't formally recognize an American's citizenship in another country, but doesn't generally oppose it, states FindLaw. Americans with dual citizenship may travel with the other country's passport, vote in both countries and run for public office. The government may renounce a U.S. citizenship if the person serves in a military that is actively hostile to the United States or commits an act of treason.