My understanding is that you can join the US forces, any branch, provided you have a green card, meaning gainfully employed for an American employer, again from my understanding. The catch is that you can‘t get any clearances above the most basic one if you aren‘t an American citizen. So this means you could join the USMC or the Army to become up to a paratrooper. But until you get citizenship, no highspeed stuff like Ranger battalion, Army Special Forces, Force Recon(Recon Bns as well I think), or Navy SEALs. Thing is, by serving in the military down there, you can fasttrack your citizen application, having to wait only 3 years, vice the normal five years. Since the usual enlistment down there is a four year contract, before you finish out your enlistment, if you worked it right, you can become a citzen, become eligable for clearance, and be in the zone where you re-up for a second enlistment, plus be fairly experienced in your service and MOS of choice, thus being somewhat attractive to the big boys assuming you are a physical stud, know your job down cold, and have you head and your life wired pretty tight. Not an altogether bad proposition, if you ask me. The only thing I haven‘t been able to find out yet is how being engaged/married affects the process, but that may not apply to you.
An intersting (to me) point, there seems to be the sentiment that you should "keep it in the Commonwealth" so to speak, rather than joining up with the Evil American Juggernaut. But, if you look at Iraq, the Brits, Aussies, and Kiwis are all there playing the game alongside the Yanks. And the SAS was in A-stan in the early stages alongside the US Marines and SF troops kicking a## and taking names. It seems to me that joining up with the Americans would be an even tighter fit than going to the Brits or Aussies. I have much more in common with the Yanks I know than the Brits I‘ve met (besides being a "subject" of HRH). America is fighting for the interests of Canada, no matter how the pinkos in the country want to spin it. What keeps America free and prosperous is what keeps Canada the same way. Besides, whenever libs scream that Canada doesn‘t need armed forces because the States will take care of us, isn‘t that proving my point? Anyway, I just found that bemusing.