Do you know that people who are not citizens of America are part of our military?

I am just wondering if people assume our military men in the US army are all citizens. A friend of mine on Facebook wrote a status that went on about a Christian America, and our citizens fighting for our country. Others below wrote about being able to read and write in English, etc. Anyway, I pointed out we have people in our military who are not citizens. I was suprised myself when I found this out many many years ago.