In early November, US president Trump raved about the "beautiful barbed wire" going up to protect MURKA from the evil furriners.
I pointed out (somewhere, I don't remember where) that it wasn't really barbed wire but concertina wire. Nasty stuff. Instead of little barbs, it's festooned with razor blades. It will cut your ass right up. When I was in Saudi Arabia, a Marine in my company stepped off a bus without looking and right into concertina. IIRC, he was cut up badly enough that he got sent home rather than returned to duty in country.
Anyway, Eric Boehm points out at Reason that the military is "securing" 1,900 miles of border with 22 miles of concertina. Or, to put it a different way, they're "securing" 22 miles of border.
But they're not actually "securing" anything.
I'm willing to bet money that Mexican hardware stores, like their US equivalents, stock wire cutters. And concertina wire cuts just like any other wire.
The purpose of concertina is to slow an attacking enemy down and or channel his approach so that you can hammer him with machine gun and rifle fire from your defensive positions behind it.
So, let's assume that the US government actually wants to defend that concertina line instead of just leaving wire there that anyone who wants to get through can cut and boogie on past. I'll try to be conservative here.
On the one hand, they wouldn't need World War 2 style concentrations of, say, an infantry company per thousand meters, along the frontal defense. The "enemy" is small groups of unarmed or lightly armed migrants who'd rather not fight, crossing presumably lightly vegetated terrain -- not exactly the Wehrmacht trying to break through US lines in the Ardennes in December of 1944.
On the other hand, that 22 miles of wire would need to be reasonably covered by effective fire.
The range of the M-60 machine gun is about 1,200 meters, but you don't want to be firing at extreme range and you want overlapping fields of fire. So let's call it an M-60, backed by riflemen, etc. every 500 meters. There are six M-60s in a rifle company's weapons platoon, so the company could cover 3,000 meters.
22 miles is about 35.4 kilometers. Let's call it 36 for simplicity's sake. You're going to need 12 Marine rifle companies -- that's four infantry battalions (or a regiment plus one battalion) of three rifle companies each, backed up by a weapons company and a headquarters company for each battalion -- to cover that territory.
So about 3600 Marines (not including regimental assets), or 100 per mile.
Assuming an average rank of very junior lance corporal (I'm being conservative, remember?), none of whom get extra pay for dependents (ha!), etc., their base pay alone ($1,931) is going to come to close to $7 million per month. That's not counting their food, their medical care, the costs of housing them, the costs of operations, etc.
Want to do that all along the 1,900 miles of US-Mexico border? Per my lowball junior lance corporal formula, the pay alone will come to a little over $600 million per month, or $7.25 billion per year.
Let's be realistic about the pay and call it $15 billion a year because most of those Marines won't be lance corporals and most of them will be getting extra allowances for dependents, etc. And that's before they eat a meal, show up at sick bay with the clap, start their Hummers up and burn gas going on patrol, etc.
I'm guessing that real "border enforcement" of that type would run $50 billion a year, easy. It would also require more than half again as many people as the Marine Corps currently has serving in all occupations, and far more than the US Army has serving in infantry.
And anyone with a pair of wire cutters and the ability to keep quiet (or a working boat) who really wanted in would probably get in anyway.