As I understand how it works, you do not need to be a US citizen to serve in the US military. Military service is a path to citizenship, but I do not believe that it is a requirement. For example, we have gotten many Phillipinos enlisting and retiring to go home to live in the Phillipines and receive their retirement pay there (one Navy CAPT I served with was involved in a fraud investigation where the family did not report the death of the retired member and kept cashing his checks).
You do not have to be a US citizen to enlisted in the armed services. However, you do have to be a US citizen to be a commissioned officer (federal Title 10 law prohibits non-citizens from being commissioned officers). Also, you cannot enlist if you are an illegal alien (though I am sure some slip through the cracks).
Oh, most definitely build the tallest wall we can!
With a minefield, moat with crocodiles, and automated machine gun placements.
Edited by DevilsAdvocate, : No reason given.