I am for rewarding/repaying/forgiving a young man who came into the US illegally but had the spirit to put his life on the line in the military for the US and what he believed as to our Constitution. I have memories of many young US citizens who declined to do such and in fact chose to go against the interests of the US.
Illegal aliens are not allowed to join the military. There is an expedited process for citizenship for green card holders who join the military. That said, they are only given limited security clearances. Certain kinds of intelligence are not permitted to be given to foreigners including green card holders.