The Union recruited foreigners during the Civil War.
How many al-Qa'ida saboteurs would it take to knife the military in the back? 100? 50? 10? Talk about a can of worms!
Or do you think that the extremely PC military, at the direction of President "Islam is a religion of peace" Bush would put up a "no Muslims or Arabs allowed" sign at the recruiting station?
Aside from the fact that it's a bad sign when you need to outsource your national defense needs, it's a very bad idea to allow people we can't screen effectively into positions of trust. There's a big difference between recruiting immigrants and recruiting foreigners, because screening foreigners will be much harder.