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I have a city that has a population of 2.9 million and an urban footprint around 2,037 square kilometres. For reasons unknown, it's been sealed off from the outside world by a shadowy military organization by disabling all internet and cell phone communications, disabling all wireless communications. A large fleet of ships blockades the city's harbour and extends along the surrounding coastline. A strictly enforced no-fly zone stops civilian aircraft flying over the area and a massive wall prevent anyone from entering or exiting the city.

The only problem is that I have no idea how many soldiers this organisation would need to efficiently maintain their occupation and keep the population in line.

How many soldiers would need to be deployed?

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closed as primarily opinion-based by kingledion, StephenG, L.Dutch, nzaman, SJuan76 Jun 22 '18 at 9:19

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ It depends on what they pack. Drones may reduce the number of soldiers needed by automatung the execution of rumaway wannabes, for example. $\endgroup$ – Renan Jun 22 '18 at 1:25
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    $\begingroup$ To add to what @Renan has said, there is also a matter of geography. Airspace is one thing; but enforcing a no-fly zone over an area that isn't directly between (say) Miami and New York may be a lot harder than enforcing one that is. Add to that, how many roads in and our are cut off by the wall? How many harbours were there? Is there a large fishing industry? Is the military letting in food? Is it an occupation or is it really a quarantine? Do they care if the people inside live or die? Lots of variables we need for an answer that's a good estimate. $\endgroup$ – Tim B II Jun 22 '18 at 1:32
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    $\begingroup$ I'm going to vote to close this as too opinion based. The difficulty in keeping 2.9 million disarmed Japanese in Sapporo under control is a lot different from keeping 2.9 million well armed people in Kansas City under control, which is a lot different from keeping 2.9 million well armed and well trained in guerilla warfare people in Kabul under control. $\endgroup$ – kingledion Jun 22 '18 at 1:45
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    $\begingroup$ @pojo-guy Modern-tech maybe, but Future-Tech has access to Hunter-Killer Drones and Mechanized Exoskeletons $\endgroup$ – Sydney Sleeper Jun 22 '18 at 2:06
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    $\begingroup$ @sydneysleeper you are working under the assumption that fear of occupiers is what makes occupation possible. In truth, it is the establishment of relationships that makes it possible. Hunter killer drones sounds good on Sci fi channel, but real life occupation requires face time. That was the number one lesson that General Petraeus taught the US in iraq. $\endgroup$ – pojo-guy Jun 22 '18 at 3:17
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Infantryman's Perspective, Eyewitness to Similar Situations

I have been witness to 3 modern day city sieges. When the taliban retook sangin in 2014, The battle of Ramadi in 2015, and the final siege of Mosul in 2017.

In all 3 cases tens of thousands of men were involved and it still wasn't enough to do what you say, and these cities were a fraction of the size of the one in your scenario. In Mosul almost 100,000 men were involved and ISIS still managed to slip the cordon. Thanks to that about 5,000 to 10,000 former ISIS fighters are living comfortably under refugee status across the continent from turkey to the UK.

My point being, the operation you speak of would require deployment of forces we havent seen since WW2. The bulk of our military would struggle to pull it off. Also, just a side note, all 3 of those city sieges I witnessed or participated in destroyed the majority of the city and displaced hundreds of thousands of civilians. City warfare is pretty brutal, requires massive force expendiature, and kind of sucks. If you can stop 80% of the enemy from escaping you did a spectacular job. Im guessing it would take 250,000 to 400,000 men and would probably not even stop large masses of people from fleeing.

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Many military manuals and studies on counterinsurgency are declassified and available online. Quick googling yields this one with a table on p. 52.

The historical data suggests 5 to 50 troops per 1,000 inhabitants. That would mean 14,500 to 145,000 troops for your fictional city. By comparison, the NYPD has 55,000 police for 8,500,000 inhabitants, or 6.5 police per 1,000 inhabitants, which serves as a reality check on the COIN data. Counterinsurgency ranges from "ordinary" numbers of police to "ten times ordinary" numbers.

It depends on just how much of a crackdown you need for your story. Iraq and Afghanistan were grossly undersized and the results were accordingly.

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Interesting question, but flawed: As history, and especially recent history proved, you can't occupy a city that does not want to be occupied. Vietnam, Iraq, Afghanistan, even the revolt of the US colonies against the English who were actually the political and military rulers! -when the population has a will to fight, they will exhaust rapidly the foreign human resources.

The real question you should ask is first and foremost based on: what kind of government is running the city you want to keep occupied? Is the population sorely tempted to go with the 'invaders'? Did they actually help them? In this case, a minimium force will do for the occupation. Otherwise you'll be needing a surging number (starting 14,500 as per answer above) and ending up in a 5x times larger number before long

More important, after the occupation you must make sure that bureaucracy runs smooth, or the occupation will end up dissolving in organizational chaos.

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