They don't talk about it but the military has to consider forcibly closing the border to prevent invasion after a mass ecological/economic disaster.

https://spaswell.wordpress.com/2016/11/18/dr-gwynne-dyer-geo...

It's long, so here's the relevant part quoted at length:

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There’s an anthropologist friend of mine, a guy called Steve LeBlanc–he runs the Peabody Museum in Harvard–who wrote a book once called “Constant Battles” about pre-civilized human beings and how they actually lived. Not the fantasy version, where they lived in, you know, the Garden of Eden and wore flowers in their hair, but the real ancient world where food shortages, including ones that you could die from, were a regular occasion. And his rule of thumb is “people always raid before they starve.”

“Raid the neighbours.”

Now, it doesn’t automatically/inevitably follow that large civilized society will do the same as small hunter-gatherer bands, but it’s the same people, you know. So the military think that there’s going to be a lot of work for them. I mean I can’t discern, and I don’t see how you would separate their motives into the ‘good ones’–they just want to be ready to deal with trouble–and the ‘bad ones’–they’re looking for a justification for their existing budget and the present justifications are getting a bit thread-worn so we need some new ones…I can be quite cynical about this on occasion; but I have taken my pills today.

But they’re right. That–I mean, you talk to the US military for example, their, I’d say, obsession in the planning. I mean, not your average grunt or his, you know, the “2nd platoon officer” but in the thinking parts of the military: the obsession with Mexico. Because there’s a 2,000 mile/3,000 kilometers (I’m in Canada) 3,000 kilometer border between Mexico and the United States. There are a hundred million people in Mexico, another 50 million south of them in Central America; the farms there are going to dry up and blow away.

There’s already a well established pattern: when you’re in deep trouble and you can’t feed your family, head north. There is an existing arrangement on the US border which might be described as “catch some, let some go.” I mean it’s–it’s not really fortified. It looks pretty impressive in the cities where people can see it, but if you drive 10, 15 kilometres out of town east or west along the border, you’re down to three strands of barbed wire and a dirt patrol track with a vehicle down it every 4 hours. It is actually meant to allow lots of people to get through because cheap illegal labor is what keeps American agro-business profitable, and the reason that it’s cheap is because the workers are illegal and they can’t bargain. So there is a reason for all this–though not one we can speak of in front of the children. This provides a safety valve for Mexicans, the Mexican state. It provides cheap labor for American farmers and, you know, the flow is maybe half a million a year, a million a year, and half of them go home again after the harvest. We can deal with that–there’s 300 million people in the United States; drop in the bucket. But try 5 million a year, because things have fallen apart and there’s no food in Mexico.

What do you do at 5 million a year? Well, the US Army’s convinced that the Congress will tell them to close that border–really close it–and they can do that! It’s absolute nonsense–though you’ll hear it often enough in the States, they go: “Ooh, I dunno…2,000 miles of border; we could never shut that–”

Bullshit.

Iron Curtain! You remember the Iron Curtain?

But the dirty secret is that you can only shut a border if you’re willing to kill people.

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