CNN reports here, that Donald Trump said

Until we can have a wall and proper security, we're going to be guarding our border with the military

Wikipedia, which ought to be as good a source as any, says that

The total length of the continental border is 1,954 miles (3,145 km).

I am not sure that this is the best site to ask, so feel free to migrate, but how many soldiers would it take to guard that wall, including support, and how much would it cost (salaries, equipment, et al)?

[Edit] after comments have pointed out that Trump was rather vague, I will state that, no matter what he means, this question is asking about securing the entirety of the border to the extent that 99% + of attempted illegal crossings can be thwarted.

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    I don't think any answer can be judged correct. It depends on how closely you guard the border. The answer could be anything, it would all depend on the assumptions you choose to make. – James K Apr 4 '18 at 8:02
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    This CNN article talks about previous deployments of troops and the numbers are relatively low when compared to the length of the border (6,000 under GW Bush, 1,200 under Obama). But Trump's proposal is (as usual) rather vague so right now there is no way of knowing the (actual, measurable) objectives nor the resources needed to achieve them. – SJuan76 Apr 4 '18 at 8:17
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    For comparison. 50000 troops from the former DDR defended the border with the FGR, a border about half the length of that with Mexico, So as an upper limit, 100 000 troops would seem to be enough to prevent an invasion, about a quarter of the National Guard. – James K Apr 4 '18 at 10:22
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    There are huge (yuge?) caveats with that comparison. Its a very different border For a start, the Mexico is not actively hostile to the US, and the DDR troops were trying to stop sophisticated people (the NATO miltiary) from coming in. I think I won't make it an answer, as it doens't actually answer the question, and depending on assumptions you could get a very different answer that would be just as good, hence to vtc. – James K Apr 4 '18 at 12:46
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    @JamesK - that's not an apples to apples comparison. DDR border needed to be defended in context of 100% prevention (98% wasn't adequate, and would be in USA), and they also served as first line of defense in case NATO rolled out (I doubt anyone expects Mochtezuma to arise again and invade) – user4012 Apr 4 '18 at 14:43

Stopping 99% of illegal crossing is something that is never going to be economically plausible (i.e. it will cost more than any economic issue caused by immigrants).

Firstly lets think about how many soldiers it will take to realistically guard a stretch of land from people crossings - to make this answerable the following assumptions are made

  • we're talking about civilians crossing unarmed without 'tactical' or other assistance
  • all parts of the border can be realistically crossed and clear lines of sign are possible between posts
  • the military are guarding it independently and assistance from other agencies is limited to after the crossing has been stopped
  • soldiers are paid at some standard rate for their time guarding the boarder and are not expecting special pay for this service
  • The objective is to catch them alive and soldiers have the appropriate tools for this including communication equipment as required
  • normal border crossing can be manned by the normal agencies responsible for this with minimal military assistance

So your going to need 1-2 soldiers every 500 meters plus an equivalent number for night time with maybe more for key times and some for lunch time lets call this 5 soldiers every 500 meters

number of stations = (3145*1000)/500 = 6290

number of soldiers = 125800*5 = 31450

so that will cover the length but there are also crossing areas given aforementioned assumptions these places should not really need any extra soldiers above what they would get using the above formula

So for 31k troops some barracks and and support staff will be needed lets say a barracks every 20km

2145/20 = 158 (rounded up)

border guards per barracks = 31k/158 = 199 (rounded down)

each barracks will have ancillary staff, its own guards and command - let say this is circa 25 per barracks

ancillary staff and guards = 20*158 = 3160

command staff = 5* 158 = 790

relatively low salary staff = 31450+3160 = 34610

basic staff salaries

private first class salary = 22629 sergeant pay = 27,338 Pay for major = 55231

lets say that the average basic pay for a soldier/ancillary worker is 25000 and for officers is 50000

basic staff pay = 25000 * 34610 = 865.25m

officer pay = 790*50000 = 39.5m

total pay = 904.75 m pa

note the average pay of a US soldier according too chron is 99k not sure how accruate this is but it would increase the cost by more than double

99000 * (34610+790) = 3504.6m

This seems like alot and may be reduced by e.g.

  • using the standing army that is already paid and staff therefore staff are already paid
  • only protecting certain parts of the boarder
  • not staffing the entire boarder all of the time
  • deployment of detection equipment

So now we have a locked down border I'm not going to guess what equipment they may desire because i really don't know and i think the above calculation are already enormously expensive. But you can be sure that the equipment will run into tens or hundreds of millions for trucks helicopters and so forth on top of this is logistics and infrastructure (e.g. building bases and roads).

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    Fantastic answer, just what I was looking for. IF you are saying $3,504.6m = £3.5b/annum, we can even compare that against the various estimates of the cost of the wall and project a break-even point (all though estimates are generally only for construction, and not for staffing or other running costs. – Mawg says reinstate Monica Apr 4 '18 at 12:04
  • "all parts of the border can be realistically crossed and clear lines of sign are possible between posts" - that's a pretty important and rather unsubstantiated assumption. What if it's only 10% of the border? – user4012 Apr 4 '18 at 14:40
  • @user4012 I agree this is a massive assumption and rather unrealistic but it's about making the question answerable. Without an assumption like this the answer could easily become an entire book or report speculating about lines of site on the boarder and which parts are passable and require guarding and so on. Also as we're talking about a single or small group of people crossing (perhaps with some basic equipment if required) and not an army this will require most of the border guarded – Steve Smith Apr 4 '18 at 14:58
  • I think you're vastly underestimating the number needed. First, you aren't going to have soldiers standing 12-hour watches, 7 days a week. You'll need at least 3 shifts per day, probably 4, plus extra for weekends, vacations, sick time... Then you're going to need more than 1-2 per guard post, else some will take bribes to let people through, or just be careless - which happens even when the people who sneak are intending to attack. And in most of that country, the guard posts will need to be a lot closer than 500m apart. – jamesqf Apr 4 '18 at 19:49
  • @jamesqf I think that my assumptions re line of sight covers some of this. I should perhaps add an explicit assumption that the guards are not corrupt and that they are not defending from attack. Perhaps the addition of a small base camp every five miles with 10-20 reinforcements would be useful (extra 6-12k soldiers). Note the whole answer is rather unrealistic it is simply an indicator.. – Steve Smith Apr 4 '18 at 20:47

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