ORIGINAL: Dave Briggs
I wasn't really trying to make a comparison but was just pointing out that most armies will accept non-citizens. In the US, anyone who is qualified can join the armed forces. As far as I know there are no units or organizations that require citizenship. That would be unconstitutional and the US Constitution affords the same civil rights to non-citizens as citizens even though I wouldn't exactly classify joining the military as a right. It's historically seen as more of a privilege. Working for military security agencies might be a bit different, but even there I don't think there is any restriction based solely on nationality. Achieving rank might be a bit different, especially in the higher officer ranks, but even this has no citizenship restrictions as far as I know. Having said that, I would be surprised if a non-citizen ever achieved the the rank of Brigadier General or higher.
Hadn't really thought about it like that but I guess the US Army has a long history of recruiting foriegners into its ranks. During the ACW whole regiments were formed of German speaking immigrants almost straight off the ships from Europe and large numbers of Irish speaking regiments were forming in New York.
Likewise in Britain substantial numbers of Irishmen were recruited into our regiments during the AWI and Napoleonic wars. And of course we still have regiments like the Ghurka's.