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As far as I can determine by my research, relative to the population, the largest (by measure of manpower) the US military ever reached was in 1945 at 12,209,238 military personnel (not limited to soldiers), which was roughly 8.8% of the US population at the time.

I would imagine this trending towards an upwards bound for a relative military size.

Although granted the nature of a human's role in the military has shifted a number of ways due to technology, I'm going to leave it fairly open ended. The candidates for this question are:

  • Any real nation (or a hypothetical nation close enough to reality)
  • Any point over the past 100 years
  • With a population of at least 15 million
  • Possessing relatively and competitively modern technology for that time

...What is the largest the active military (not in reserves or military just in a book[*]) could hypothetically be by percentage of the population? What factors would allow a military of this size? How stable would a military of this size be, logistically?

I'm fine with adjusting politics, economics, geography, and culture to achieve the 'perfect storm,' as long as these adjustments have some root in real life historic examples.

[*]If the people are currently serving in other professions outside of the military I would not consider them part of the military; their active profession should be part of the military. Further, people who previously served in the military but no longer serve would also not be considered part of the military.

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    $\begingroup$ Related, only difference is medieval versus modern: worldbuilding.stackexchange.com/questions/20826/… $\endgroup$
    – James
    Jul 25, 2016 at 15:42
  • $\begingroup$ @James Useful, and thanks for the link. Still I'd imagine there'd be a massive difference between 1910s-2010s and 400s - 1500s. $\endgroup$
    – Ranger
    Jul 25, 2016 at 15:43
  • $\begingroup$ Nex can you define a timeframe a bit more? The nature of warfare has evolved DRASTICALLY in the last 100 years. WWI required masses of poorly trained troops, today we need fewer troops to apply the same force but they are better trained and more expensive. Massing troops like we did in WWI would do no good today and leads to nothing but more unnecessary casualties. $\endgroup$
    – James
    Jul 25, 2016 at 15:49
  • $\begingroup$ @James To answer your question indirectly, for my story (I know this sounds odd) I'd be fine with the person answering to pick the decade during that timespan that allows for a perfect storm (as long as they address why they picked that decade in the question). Or to rephrase: I'd find any decade in that timespan acceptable for purposes of my world-building. $\endgroup$
    – Ranger
    Jul 25, 2016 at 15:51
  • $\begingroup$ For pillaging tribes (Huns? Mongols?) probably reaches 100%. However the more advanced the warfare the less people are on the frontlines and more on domestic front (assuming population willingness to go 100% war economy). On another part of spectrum there might be war fought only with autonomous drones with rest of population being in supply lines (farms, factories...) with possibly handful of generals - and that possibly constitute "15 minutes into the future" type of Sci-Fi. $\endgroup$
    – Maciej Piechotka
    Jul 26, 2016 at 4:16

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The USSR in WWII drafted something like 35 MILLION people. That is out of a population of about 200 million. They easily beat out the USA for total population engaged in the war. Generally, it helps to have a totalitarian government if you want to push the upper bounds of total population under arms. North Korea backs up this trend. They claim to have about 25% of the total population of North Korea militarized. Of course, this level of mobilization comes at a high cost. They are also starving to death regularly (dependent on Chinese food handouts) and have constant, rolling power outages.

Historically, some societies were geared to mobilize a higher percentage of the total population than modern civilizations. The Mongols were good at this. Virtually every man in a tribe was a warrior, and the tribes could be mobilized into units and controlled the way a conventional army would be. They could get away with this because the Mongols brought their logistics along with them across the steppes: big herds of animals from which they got almost 100% of their daily needs. Virtually everything they used or ate came from the horses and livestock who came with them, so the entire nation basically fed off grass.

The USA itself beat out it's WWII percentage of mobilization during the Civil War. Specifically the Confederate side achieved an impressive percent of the total population serving in the war. They had an estimated 1,082,119 soldiers out of a population of about 9 million. That's a pretty high percentage.

Obviously a different question would be numbers versus effectiveness, because the side who mobilized most is not usually the side who WON in history.

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  • $\begingroup$ Can you confirm that the USSR had 35 people active in the military, and not reserves? If you can confirm that I'd say this is currently the leading answer. $\endgroup$
    – Ranger
    Jul 25, 2016 at 20:17
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    $\begingroup$ The Soviet Union had ~35 million total soldiers over the course of a four year war with almost 10 million deaths. Nearly 30 million of those were conscripts, which took time to field once the invasion began, and there were millions killed in the first year alone. It's unlikely that they had that many active at any one time. However, given the OP's question I think the WWII Soviet Union is still the best historical example from the 20th century. $\endgroup$
    – Avernium
    Jul 25, 2016 at 21:13
  • $\begingroup$ @NexTerren They were. Non-mobilized reserves are a feature of peacetime armies, you don't engage in mass conscription until you've fully mobilized the reserves first. If anything as it stands the USSR was probably slightly over-mobilized; in that lend lease aid replaced what would otherwise have been a decent chunk of manpower needed for manufacturing of war materials. Highlights of what the USA provided were a large majority of the trucks, locomotives, and rail cars used to supply the red army; along with enough Sherman tanks to equip several armored divisions. $\endgroup$
    – Dan Is Fiddling By Firelight
    Jul 25, 2016 at 21:17
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I can't remember my source, but I do remember reading up on this subject a few years ago.

The main conclusion was that the maximum size military force that a "regular" economy can sustain is around 10% of their population, reaching as high as 15% in war time.

Quite simply, every soldier on active duty is one less able bodied, in their prime, person not contributing to your economy. These people need plenty of resources, however, which means that many others must now be tapped to provide basic needs for them rather than manufacturing luxury goods, or generally improving society (pursuing arts, science, etc.)

Also keep in mind that for every soldier on active duty the number of support staff increases exponentially. Each soldier needs people to do his laundry, cook for him, repair or replace his equipment, fix or build their vehicles, etc. (imagine the number of support engineers and the size of the industry providing spare parts, etc. for an armored division).

Some critical factors affecting this percentage is going to be the level of automation of your manufacturing facilities, as well as the amount of food you can grow, and - again - how automated that process is as well.

Generally speaking a society will break these standards if they're facing a terrible threat (when fighting for their very survival it's every man on deck), or if the regime wishes to keep everyone under their tight control through military occupation. However, in either of these scenarios the society in question ceases to be a truly productive one.

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  • $\begingroup$ Silly question, but by "10%" and "15%" do you mean 10-15% of the total population? $\endgroup$
    – MozerShmozer
    Jul 25, 2016 at 15:48
  • $\begingroup$ @MozerShmozer - yes, I'll edit $\endgroup$
    – AndreiROM
    Jul 25, 2016 at 15:50
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I think we can get a good answer by looking at the real-world situation of Israel, where every able-bodied citizen within a certain age range is required to serve for a period of time in the nation's services. (Although there are ways to fulfill this obligation other than military service, I think that overall this should give us a realistic estimate).

I found these statistics that give an Isreali population of 8.05 million, with a total number of active and reserve personnel at 0.79 million. Let me take a second with my calculator and...

9.813%

Let's round that to 10%, which agrees with other comments and answers.

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Let's take your number of 15,000,000 and apply this link's reasons for reduction using metrics from the US and our laws...

People under 18 = 24%
People over 65 = 14.9%
People with disability = 8.5%
Exempt skills/professions = I don't know
Outlaws, criminals, dodgers = 10% (depressing i know)
total = 57.4% = 8,610,000
Remainder = 6,390,000

Let's assume a 50/50 split of men/women. ~10% of military are women which means 1 woman for every 9 men So we just divide the above by 2 to get the max male military (3,195,000)
And then divide by 9 and add that (355,000) to arive at our max military population. (3,550,000)

This ends up being 23.6%

We can reduce this for economic reasons and food production reasons, but I don't have the numbers for that so I can't say. Also this is presuming that those exclusions won't join in on combat, when we know that when push comes to shove at least a few will, likewise, a portion of these will be needed for several other things, like like policing, medical, construction, manufacturing, etc even with farming techniques that makes providing for very large populations a negligable portion of the population.

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  • $\begingroup$ Typically it's best to avoid one line or hyperlink answers. Perhaps you could summarize the article in your answer? $\endgroup$
    – MozerShmozer
    Jul 25, 2016 at 15:46
  • $\begingroup$ Durakken, link only answers are frowned upon. If the link dies in the future your answer is no longer useful, please pull the relevant info out of the link, cite your source and provide the link as reference material. $\endgroup$
    – James
    Jul 25, 2016 at 15:46
  • $\begingroup$ First, avoid simply providing links in your answer. Links can die, and your answer will no longer be useful. Second, your link addresses an era considerably outside the timeframe of the last 100 years. $\endgroup$
    – Ranger
    Jul 25, 2016 at 15:46
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    $\begingroup$ Your percentages won't be simply additive, since people with disabilities and criminals can be over 65 or under 18. $\endgroup$
    – ckersch
    Jul 25, 2016 at 16:48
  • $\begingroup$ @ckersch True... but that skewing is more than made up for by the fact that the over 65 number should be more like over 40. $\endgroup$
    – Durakken
    Jul 25, 2016 at 16:50
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The further in the future the better.

Militaries are a drain on a countries economy (usually):

  • Militaries, while useful and sometimes necessary are inherently a drain on the economy because you are taking able bodied people and supporting them rather than having them work to produce goods and services. These are just a few of the things you have to provide

    • Food
    • Supplies
    • Transportation
    • Medical
    • Housing
    • Training

  • The hard limits on sustaining a standing military are the logistics behind supporting people who are not producing anything for the economy.

So why is the future better?

The more advanced technology becomes the fewer bodies you need to provide the basic supplies for human survival. The most obvious example is farming. What used to require hundreds of people can now be done by one guy and his big damn tractor.

A portion of the population is absorbed into the support system for creating said tractor (engineers, welders, etc etc etc etc) but not all of them.

Tech can be a pain but it does make us more efficient and the more efficient we become the more bodies we can support in the military.

A conflict...

So. While increased technology makes us more efficient and allows for a larger military...it also reduces the need for manpower in the military.

As weapons evolve and become ever deadlier the masses of infantry become increasingly unnecessary.

So while we have the ability to support a larger military there is less for people to due thanks to automation and technology.

In short, given the level of technology you make available in your world, the military can become basically any percentage of the population that you want.

Without knowing your particular scenario its very difficult to give you a guess on what percentage that would be.

You could also, if you were so inclined, create a society that is nothing but a big military. Militaries can be self sufficient, there is nothing to stop a grunt from building roads and doing farming while they aren't being shot at.

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  • $\begingroup$ I feel this could be rectified in WWI-2 levels of technology if airplanes/blimps/rockets were not available. The inability for strategic level weaponry forces an emphasis on manpower, trains/supply chains as a the main factor in strategic decisions. $\endgroup$
    – knowads
    Jul 25, 2016 at 18:28
  • $\begingroup$ I disagree that the future is better. I included the Mongol Empire in my example because they were able to achieve a MUCH higher percentage of total population mobilized for war than any modern, 20th or 21st century society. It depends on how a civilization is organized more than the over all technological level. $\endgroup$
    – JBiggs
    Jul 26, 2016 at 17:44
  • $\begingroup$ @JBiggs Please read the last couple sentences of my answer. $\endgroup$
    – James
    Jul 26, 2016 at 17:50
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Current strength of US of Israel armies pointed in other answers can be good estimations of how big can be a standing army. However, in historical wars the percentage of population mobilized has arrived to very higher percentages. According to Wikipedia article on Paraguayan War Paraguay mobilized about 150.000 men out of a population of about 500.000 or less. That is 30%.

An another historical example: I can't remember the source, but at the end of the Roman Empire, Germanic peoples could mobilize a warrior out of 4 people. That makes 25%.

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  • $\begingroup$ Israel is unique in that most of it's military personnel have professional lives outside of the military. They don't actively serve. I'm concerned with a standing, active army. $\endgroup$
    – Ranger
    Jul 25, 2016 at 20:34
  • $\begingroup$ The Paraguayan War is an example of an unsustainable military force. By the end of the war, most of Paraguay's men were dead. Afterwards, the society resorted to legalized polygamy to support the widows and rebuild the population. $\endgroup$
    – Jasper
    Sep 6, 2018 at 7:15

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