U.S. Army to Recruit Ugandans.

No, Thorin, citizenship is not automatic. They have to go through the same lengthy and convoluted process that all naturalized citizens do. They apply and wait, apply and wait, apply and wait, all while serving in uniform. I knew quite a few in the Marines.

I've read of Mexican nationals serving with the Marines who died in combat have have been granted citizenship posthumously.

There was also a brief international incident when the family of a deceased Marine/Mexican national requested a military funeral...in Mexico.

Given the history involved, the Mexican government doesn't like armed US Marines on its soil. In the end, the problem was resolved and the Marine was buried.