UCAS has had some pretty tense relations with many of it's neighbors in the past: war with the Tir, war with the NAN, impending threats (percieved or real?) with Saito in California...

So what would the borders with those countries look like? On one extreme, we have the border between East and West Germany during the cold war, with hundreds or thousands of miles of fencing, military guards, and reaction forces at the ready. Somewhere in the middle you have US-Mexico border, with a cursory inspection at the major crossing points, and some attempts to nab smugglers and coyotes, but mostly ineffective. At the other end of the spectrum, many times in Europe you might cross from one country to another without even realizing it.

Which do you think the UCAS-NAN borders would be? How about UCAS-Tir? UCAS-CAS? CAS-Atzlan?

Well they get on pretty well with the CAS so I figure it's probably be about on par with today's current measures along the border with Canada. For the Sioux and AMC, again no real hostilities as such. Probably wont be massively invasive but I could see them pulling over a fair few people and checking details. Quebec is an ultra-nationalistic paranoid country so border patrols are very tight at least on their side and shooting to kill on those that don't stop the norm.

The UCAS doesn't have a border with the Tir since Seattle is completely surroudned by Slaish-Shidhe land. And has the UCAS ever actually been at war with them? I know that California has, but they were independent and the CFS at that point.

On the CAS/Aztlan border I'd think it was pretty much shut down with no traffic going through. The CAS still considers it to be an hostile enemy occupying their land and the Azzies would like tp grab anything they can, never mind recent events in Denver. Plus have opposing armies staring at each other over the border playing who blinks first means it's pretty tight. Smugglers do still get through though and non-CAS traffic could probably make it using main routes but Aztlan has fairly strict immigration policies.

Well, if the fact that there are (and have been for years) Marines on the U.S./Mexico border is any indication, the UCAS/Sioux border's going to be almost as militarized as downtown Austin.

There are? I thought they got rid of them donkey's years ago? Something about Posse Comitatus not allowing it, although I could well be mistaken. But even then, aren't they more of an 'Oh shit!' backup thing? Not like you'll see them manning the border checkpoints.

Edit: My apologies. Having just looked at Target: Smuggler Havens, either the military got in on the act or the border guards got seriously militarised- talks of Banshees and Yellowjacket hellicopters consistently being used in border patrols. And it seems they have the National Guard, never mind regular military involved. So upgrade the Sioux/AMC borer to how the US/Mexico one is now I think if a little more cranked up.

Posse Comitatus would only apply if the Marines are used against US citizens. Mexicans, or any other nationality for that matter, are fair game.