The Immigration and Nationality Act (I.N.A.) allows people born in other countries to gain U.S. citizenship through U.S. military service—in some cases without going through the usual preliminary step of getting a U.S. green card (lawful permanent resident). This is a highly unusual possibility; almost no one else can apply for citizenship without having held a green card for several years. The exact legal requirements depend on whether you served in the military during peace or war time.
Military members can take steps toward U.S. citizenship by serving in either the U.S. Army, Navy, Marines, Air Force, or Coast Guard, or in a National Guard unit while the unit was federally recognized as a reserve component of the U.S. Armed Forces. (See the U.S. Code of Federal Regulations at 8 C.F.R § 328.1.)
The law does require you to complete some minimum amount of service before you become eligible for U.S. citizenship. Exactly how long depends on whether you serve during peacetime or during a war.
During peacetime, foreign nationals who serve at least one year in the U.S. military must get U.S. lawful permanent residence (a green card) before they qualify for U.S. citizenship—but they do enjoy one important advantage. Instead of waiting until they've held their green card for five years before applying for citizenship, they can apply one year after receiving the green card. (See § 328 of the Immigration and Nationality Act, or I.N.A., or 8 U.S.C. § 1439.)
Some other conditions apply. The applicants' service must have been considered honorable. They must, like everyone applying for citizenship, be at least 18 years or older, of good moral character, able to show knowledge about American history and government and the English language (written, spoken, and read), and demonstrate an attachment to the U.S. Constitution.
Unlike other naturalization applicants, you won't have to pay the N-400 application fee. But you will have to complete and file USCIS Form N-426, Request for Certification of Military or Naval Service. This will require input and a signature from a U.S. military official.
What if you have already been discharged from the military? The discharge must have been honorable, and you face a deadline: If more than six months pass after your discharge and before you file your application for naturalization, you will be back to having to complete five years as a green card holder before applying, just like other civilian applicants. But keep reading: The next section of this article describes extra benefits to those who served during wartime.
If you enlist in the U.S. armed forces during wartime, you can apply for U.S. citizenship after as little as one day of service. (See I.N.A. § 329, 8 U.S.C. § 1440.) Various periods of wartime count, most recently including the time that began September 11, 2001 and will end whenever the U.S. President announces a cease to the hostilities.
Applicants under this section of the law must meet most of the same requirements as any other applicant for naturalization. These include being able to read, write, and speak English, having good moral character, being able to pass a test on American history and government, and (after approval) swearing an attachment to the U.S. Constitution. However, you will not be held to the usual requirements regarding age and length of time as a permanent resident living in the United States.
You can apply for citizenship under this section while you are overseas.
The process of applying for naturalization beings by sending an N-400, Application for Naturalization, to USCIS. See Five Tips for Filing the N-400 Form. The USCIS website contains useful information on preparing for the exams.