While there is a statutory requirement that only a United States Citizen may become a commissioned officer, this is not true for enlistment. Certain non-citizens can enlist in the United States Armed Forces. To be eligible to enlist, a non-citizen must:
(1) Entered the United States on a permanent residence visa or has an Alien Registration Receipt Card (INS Form 1-551/I-551 greencard or stamped I-94), and
(2) Established a bona fide residence, and
(3) Established a home of record in the United States.
(4) The visa and/or "greencard" must have sufficient time remaining on it (expiration date) to be valid during the entire term on enlistment. While non-citizens may enlist in the U.S. Military, they are not allowed to re-enlist (stay in beyond their first term of service), unless they first become U.S. Citizens. However, after serving for three years, any additional residency requirement for citizenship eligibility can be waived.
I should note here that the United States Military cannot and will not assist in the immigration process. In order to join the U.S. Military, one must legally immigrate first, and then apply to join the military.
For enlistment purposes, citizens of the United States include citizens of: Guam, Puerto Rico, The U.S. Virgin Islands,The Northern Marianas Islands, American Samoa, The Federated States of Micronesia, and The Republic of the Marshall Islands.
Not all non-citizens may be eligible to enlist. Applicants who have been residents of countries considered hostile to the interests of the United States require a waiver. [snip]
While non-citizens may enlist, they will find their job choices extremely limited. DOD policy prohibits granting security clearances to non-U.S. Citizens. Therefore, non-Citizens. who enlist in the United States military will be limited to those jobs which do not require a security clearance.