I have no objection to foreign born naturalized citizens. But not foreigners. I would not be comfortable with Somalis and others from enemy countries signing up. Perhaps it is because I know of cases where the motivation for foreigners was not love for this country but personal gain and worse, etc.
Foreigners, as you define them, can’t enter thye US military. If they are green card holders, i.e., legal permanent residents, they can. Currely, 35,000 serve in the military and can apply for expedited citizenship. I gather you would like to prohibit green card holders from serving.