A person might actually acquire U.S. citizenship at birth regardless of the country in which they were born.
It is possible for a person to automatically acquire U.S. citizenship despite not taking any action of their own.
A “Consular Report of Birth Abroad” may be issued by a U.S. Embassy abroad to a person that is under 18-years-old.
A permanent resident may apply to be a naturalized citizen of the U.S., thereby allowing them to enjoy all of the benefits that come with U.S. citizenship.