In order to join the US Military, you must either be a US citizen, or you must be a legal permanent immigrant, physically living in the United States, with a green card. The US military cannot and will not assist with the immigration process. If you are not a US citizen, you must legally and permanently immigrate to the United States first, via the regular immigration procedures and quotas, establish a residence, and then (if you meet the other qualifying criteria), visit a military recruiter's office and apply for enlistment.
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 legal immigrants may be eligible to enlist. Applicants who have been residents of countries considered hostile to the interests of the United States require a waiver. See your local recruiter for the most current list of countries considered hostile to the interests of the United States.
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.
For legal immigrants who do enlist, there are accelerated citizenship procedures for non-citizens on active duty. For details, see our article, Becoming a Citizen in the US Military.