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Suppose you want to invade Russia. But, beating it in a conventional or nuclear war isn't possible for your people for some reason.

What if, instead, you line up 500,000 soldiers to move to Russia with civilian cover stories, blend in, and at the right moment, emerge everywhere at once with the benefit of surprise, and take over, dropping their cover and becoming an invading/occupying Army (a bit like the lizard people TV mini-series V, but with humans, not lizard people). Put another way, something a bit like a supply it yourself Fifth Column.

Could it be done?

How quickly could it be put in place without being discovered?

What tactics or tools would be needed?

Are there any historical precedents for similar tactics being tried and either working or failing?

(Of course, Russia is just a concrete example. It could be India, or Brazil, or Nigeria, instead, for example.)

Short answer is no, but the longer answer is far more interesting.

First of all, if you're looking at a simple, knock down, drag out fight with the military in question, let's assume you need equal numbers to the military in question.

There is a really cool graph out of the world bank that shows the percentages of population in the military across the world, and by country, over time. The current average is around 0.9%, but let's assume that you want to get 1% of Russia's population being made up of your fifth column.

Russia's population is currently around 145m, meaning you need 1.4m, maybe 1.5m sleepers in the country. Based on some statistics I pulled off the internet, in 2017 Russia admitted around 250k immigrants that year.

Based on this model, assuming you can take EVERY immigration place in the Russian admission program, it's still going to take you 6 years to embed your sleeper force.

BUT; if you want an army and not a network of terror cells, you also need equipment. You need guns (of course), possibly tanks, planes, missiles, and LOTS of computers. You need infrastructure like training bases, etc. Otherwise, all you're doing is engaging in a guerrilla war in the streets, and that's still going to need lots and lots of small arms.

BUT; you also need a plan. Just going street to street and taking the country is (in a word) dumb. You want to take critical infrastructure, especially communication infrastructure, and neutralise your enemy's capacity to fight back. That means taking their bases et al. For that, you really want to embed your army in their army.

That means infiltration, having kids that are born in Russia, and getting them ALL to join the military. That's on average a 30 year investment or so to get the outcome you're after.

I'd argue that there's more efficient ways to do it, but it does have one single benefit that ties in nicely with Sun Tzu; you'd have your army literally living off the enemy lands once you've started your covert invasion.

Russia has one of the most liberal immigration policies in the world (a measure Putin's government implemented to counteract its plummeting population due to demographics), but a half-million people in a short period of time is a bit excessive. Still, somewhat theoretically possible.

There are two problems you're going to have to deal with. Russia has one of the strictest gun control regimes on the planet. Aside from shotguns and rifles that people in the rural area have limited permission (note: not right) to possess, there are no legal privately owned firearms. So where this army getting its weapons? And where are they keeping them?

The second issue is maintaining secrecy. There's actually math that can be used to calculate how long a secret can be kept before it can be expected to get out.

On the Viability of Conspiratorial Beliefs

Conspiratorial ideation is the tendency of individuals to believe that
events and power relations are secretly manipulated by certain
clandestine groups and organisations. Many of these ostensibly
explanatory conjectures are non-falsifiable, lacking in evidence or
demonstrably false, yet public acceptance remains high. Efforts to
convince the general public of the validity of medical and scientific
findings can be hampered by such narratives, which can create the
impression of doubt or disagreement in areas where the science is well
established. Conversely, historical examples of exposed conspiracies
do exist and it may be difficult for people to differentiate between
reasonable and dubious assertions. In this work, we establish a simple
mathematical model for conspiracies involving multiple actors with
time, which yields failure probability for any given conspiracy.
Parameters for the model are estimated from literature examples of
known scandals, and the factors influencing conspiracy success and
failure are explored. The model is also used to estimate the
likelihood of claims from some commonly-held conspiratorial beliefs;
these are namely that the moon-landings were faked, climate-change is
a hoax, vaccination is dangerous and that a cure for cancer is being
suppressed by vested interests. Simulations of these claims predict
that intrinsic failure would be imminent even with the most generous
estimates for the secret-keeping ability of active participants—the
results of this model suggest that large conspiracies (≥1000 agents)
quickly become untenable and prone to failure. The theory presented
here might be useful in counteracting the potentially deleterious
consequences of bogus and anti-science narratives, and examining the
hypothetical conditions under which sustainable conspiracy might be
possible.

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0147905

TLDR version: the more people involved, the faster someone will blow it. Using some of the examples in the article, counting just the 500,000 soldiers it will be a near certainty the operation will be blown in under 3 years. There's 50% odds it will be blown within one year. And that doesn't count the people staying at home who know about the operation.

So, realistically, no.

The biggest problem that you're going to face is secrecy

I probably don't need to point out that 500,000 people is a large number. Each one of those soldiers is a detection risk. If any one of them acts suspiciously enough to be picked up by the Russian intelligence operations. Even worse, each one of your soldiers is a defection risk.

And once you've lost the element of surprise, every part of your plan works against you. Your soldiers are isolated, difficult to contact, and in a foreign country. They'll be easily picked up one by one, and by the time it comes for you to trigger the attack, you'll have no army.

Even if every member of your army has ironclad loyalty and master level acting skills, you'll still get discovered

Wikipedia places the population of temporary migrant workers at about 7 million. That means that 500,000 additional people is about a 7% increase in that count, not exactly a drop in the bucket. Especially since the increase is coming entirely from a single country.

Even if some of your soldiers arrive through other channels, you simply can't conceal the movement of that many people, and the powers that be are going to notice. Especially when they notice that your armed forces are vanishing mysteriously.

Infiltration might work for small targets, but a whole country is too big of a chop for it to work alone.

First of all, if you want to be secretive, you cannot board 500000 soldiers on the smallest number of flights, so you would have to dilute them among normal passengers. Let's say you use 50 soldiers per flight. It takes 10000 flights to carry the entire group, and assuming you fly to the major 3 cities, with a flight every day, it will take you a bit more than 9 years just to infiltrate them.

Then you need to provide them logistic support: they will have to be provided weapons, instructions, means of sustaining, a believable façade to ensure they can stay that long in a foreign country without looking too suspicious and without leaking any info outside or defecting. Oh, of course you need that façade also before they depart. Formally they never worked for the army (and I doubt your own government will be happy with having 500000 secret soldiers).

Then, when the day X comes, you cannot hope for them to take over the country without giving them some sort of support: air supremacy is a must in modern warfare, and you have infiltrated only soldiers.