Green Card

I am not an American but as far as I understand it the Green card is offered to foreigners who have skills in jobs that cannot at the moment be filled by Americans. Highly skilled people with a specialty and excellent command of English often qualify. Once a foreigner has a green card he or she can legally work in the USA but the person is still considered "an alien" until the person is eligible to apply for citizenship. I don't know after how many years one can apply. To my knowledge it has nothing to do with with the armed forces and holders of green cards are not recruited into the army unless the job for which they obtain the card is actually in the army.