The bugeisha (warrior lady) was typically the of a high caste warrior family. The tradition predates the establishment of a formal samaurai class, but persisted until at least the late 19th century.
In a time when men were frequently killed in battle or other dangerous occupations, or simply absent for prolonged periods of time to provide for their families (fishermen, hunters), it fell to the women to take up arms and defend the villages.
The upper classes were expected to lead and participate in battle, and to participate actively in law enforcement. The absence of a man did not alleviate the high class family's duty to lead and defend.
Japanese culture was, by our standards, very brutal. For example, in training a child in swordsmanship, at about 12 years of age they would be taken to the prison to practice getting the right angle to make a good cut on a human body.
It was in this environment that the high class lady was expected to hold her own and prevail over criminals, and to lead militias and armies. Needless to say, with the difference in sheer strength between men and women, their training was less compromising than that of their male counterparts.
Edit: Bugeisha in popular entertainment
The anime "Yona of the Dawn" presents the Japanese cultural mythos of the bugeisha in a popular entertainment. It's the classical story of a spoiled teenage noblewoman caught up in palace intrigue, who escapes and then returns for revenge/justice. While there may have been women who fit this mythos, most of the biographies I have read of bugeisha's indicated that they we nobility that began harsh training at an early age with the intent that they would fight in and/or lead military/police actions.
2Except male pregnancy? Because one man can have 1000 kids in a year, when one woman is limited to one. That's what promotes groups with male warriors. Good thing to rebuild nation, to save women. Beneficial, evolutionary. – Mołot – 2018-01-27T20:26:17.827
2>Horses. 2) Methods of production that encourage egalitarian gender roles, like horticulture. 3) A very particular social history, see Dahomey. 4) Easily available natural birth control, like silphium. 5) Horses.
< – Random – 2018-01-27T20:31:37.333
1If your setting incorporates it, gender neutral or even female specific magic can also get you far. – Random – 2018-01-27T20:41:50.977
3Molot's comment probably has some statistical validity (though one should avoid evolutionary determinism; many countries today have increasing numbers of female soldiers, such as 30% in Eritrea). It is likely less valid in cases of high indiscriminate violence against a group, so that's a factor you might consider adding for realism. – Obie 2.0 – 2018-01-27T21:06:58.757
4And one really shouldn't ignore cultural or situational factors. Some insurgent groups, such as FARC and the PLA of Nepal, have very high rates of female soldiers (perhaps higher than 40%). Besides making them more effective at potentially achieving their immediate goals (more soldiers is always a bonus), I suspect the influence of certain Communist notions of strict or even mandated gender equality shouldn't be discounted. PLA has a 30% female quota, for example. – Obie 2.0 – 2018-01-27T21:10:37.107
Lack of men would certainly be one potential reason. (Think Amazons) – JustSnilloc – 2018-01-27T21:42:34.443
I'm a little late to point this out, but the other question asks for and received virtually all biological answers. This question is more focused on culture, and therefore really shouldn't be considered a duplicate. – Random – 2018-01-28T05:39:31.097
OP, another rational (also involving horses) that I just remembered is the one from the tabletop RPG Reign, which posited a culture in which a man riding a horse was equated with castration, and looked upon much as man running around in a frilly tutu might be today. (Not that there is anything wrong with frilly tutus...). The writer used this to justify female cavalry. – Random – 2018-01-28T05:43:28.807