Although the Army wants people from all different backgrounds and experiences, all candidates must be a U.S. citizen or permanent resident with a valid Green Card (officially known as a I-551 Permanent Resident Card).If you already have a Green Card, serving in the Army can reduce the residency requirement for becoming a U.S. citizen to as little as one day instead of five years. The naturalization process for citizenship can begin as soon as your first day of Basic Training.