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.