I just finished the game but couldn't conquer both Aztec and Capala ( or something like that)
My question is, how to double-cross them? I mean, what's the requirements, because I kind of made a promise with my expeditions about conquer them both and want to try that in the 2rd playthrough :))

Thanks man. Gonna try that.
But where is Cpt.Noriega? And how did you found out about this requirement? Did the game mention it somewhere ?

It's implied more than directly mentioned. When you visit your Barracks it'll tell you stuff like "You have a sizable force, but not enough to face the Aztec Empire" and such.

Well, I got the option to double-cross my allies at the end from Noriega (in my case I had crushed the Aztec Empire, so I had to talk to him in the Aztec capital's temple). I looked up the forums to see if I had missed anything, and saw that to get that option you need the full garrison.

Which wasn't a problem for me since I started construction pretty early, and did most of my exploration after the fortress was built up. Plus the game goes out of its way to tell you that you should have a large garrison, so I tried to max it ASAP, since I thought there would be random attacks on it or something.