Enlistment into any branch of the US Military, by citizens of countries other than the United States is limited to those foreign nationals who are legally residing in the United States and possess an Immigration and Naturalization Service Alien Registration Card (INS Form I-151/551 - commonly known as a "Green Card"). Applicants must be between 17 and 35; meet the mental, moral, and physical standards for enlistment; and must speak, read and write English fluently.
The US Military branches cannot assist foreign nationals in obtaining admittance into the United States. Questions concerning immigration to the United States should be asked of the US Embassy. Only after immigration procedures are completed and an applicant is legally residing in the United States may an application for enlistment be accepted.
The US Government agency which is responsible for immigration and naturalization is the US Immigration and Naturalization Service on:
http://www.ins.usdoj.gov/