According to Chron, uncle Sam wants you -- as long as you are a U.S. citizen or a permanent legal resident of the U.S. Non-citizens can join the U.S. Army as an enlisted soldier, providing they meet specific legal immigration requirements. Non-citizens are not eligible to become commissioned officers in the U.S. military, and they may also be barred from certain Military Occupational Specialties, or job categories; particularly those requiring security clearances.

Step 1Be a permanent legal resident living in the U.S. with permission to work in the country. Neither tourist nor student visas are sufficient for enlistment in the U.S. military, and undocumented immigrants are not eligible to join the U.S. military. Non-citizens must also meet the same standards of physical fitness and education as U.S. citizens seeking to enlist.

Step 2Have a valid Permanent Residence Card -- form I-551 (popularly known as a Green Card) -- to prove your legal resident status. If you don't have a current I-551, you have to show proof that you have applied for renewed status, such as an original receipt from the U.S. Citizenship and Immigration Services showing you have paid for the renewal application -- form I–90 (Application to Replace Permanent Resident Card).

Step 3Apply to join the enlisted ranks of the U.S. Army. You will not be permitted to begin formal military training until your legal permanent resident status is established. Your Permanent Residence Card must be valid for the duration of your enlistment.

Step 4Apply for U.S. citizenship if you want to be eligible to reenlist beyond your initial service commitment or to become a commissioned Army officer. Although the normal path to citizenship requires a legal resident to maintain that status for at least five years while living and working in the U.S., the military offers an accelerated program that takes about 10 months.
source:http://www.idonshayo.com/how-to-enlist-in-u-s-a-army-if-you-are-not-a-u-s-citizen/