In a moment of distraction when I first read this my thoughts were of 'ET' type aliens, and I wondered how some might legal and others illegal. But then the penny dropped.
The idea of loads of troops sounds fine, but you still have to train, equip and provide ongoing logistical support, etc., etc. And having an occupation force anywhere were the locals are in active resistance sounds to me like something to be avoided if at all possible.
Anyway didn't you guys snatch a load of territory in - what was it - 1848? So perhaps some hispanic folks are merely going were they feel they may have an inherent right to go?