Alaska Jim,
My son did two tours in Iraq as a Marine and spent a total of 9.5 years in the service. We signed the papers so he could join on his 17th birthday. Special permission had to given because he did not have a diploma but he had a GED and 16 college credits.
During his time in the service, he served with many fine Marines who were from other countries and who were not yet citizens. Some of those fine young Marines gave the ultimate sacrifice for country they were not yet citizens of. 
When I worked, we had VISTA volunteers or Americorps volunteers. Several of them were not citizens of the U.S. at the time of their volunteering. 
Years ago, 1965 or so, my parents sponsored a young Cambodian woman who had left her country for political reasons. She had been sitting in a jail in Vietnam for six months until former employer (publisher of a anti-government paper) who also had left, could arrange away to get her to the US. My parents sponsored her and she came, escaping POL POT.