Sometime back, I had posted What does the military history literature say about the effectiveness of civilian ownership of firearms in both preventing and defending invasions ? on r/WarCollege and also on r/progun and received some good replies. I’d suggest you read both threads. I couldn’t find any literature on widespread ownership of firearms helping civilians defending against invasions but recently, I found a great (but incomplete) manuscript, Carroll Quigley’s Weapon Systems and Political Stability: A History (pdf), that studies the history of mankind since pre-historic period till the 20th century (even though the book only covers the history till 1500 AD because Quigley died while he was preparing his manuscript) and concludes that “the dominance of democracy in the 20th century is attributable to the acceptance in the 19th century of a weapons system that favored democracy, the hand gun and rifle.”
An excerpt from his other book Tragedy and Hope (pdf) on the same topic:
On the military level in Western Civilization in the twentieth century the chief development has been a steady increase in the complexity and the cost of weapons. When weapons are cheap to get and so easy to use that almost anyone can use them after a short period of training, armies are generally made up of large masses of amateur soldiers. Such weapons we call “amateur weapons,” and such armies we might call “mass armies of citizen-soldiers.” The Age of Pericles in Classical Greece and the nineteenth century in Western Civilization were periods of amateur weapons and citizen-soldiers. But the nineteenth century was preceded (as was the Age of Pericles also) by a period in which weapons were expensive and required long training in their use. Such weapons we call “specialist” weapons. Periods of specialist weapons are generally periods of small armies of professional soldiers (usually mercenaries). In a period of specialist weapons the minority who have such weapons can usually force the majority who lack them to obey; thus a period of specialist weapons tends to give rise to a period of minority rule and authoritarian government. But a period of amateur weapons is a period in which all men are roughly equal in military power, a majority can compel a minority to yield, and majority rule or even democratic government tends to rise. The medieval period in which the best weapon was usually a mounted knight on horseback (clearly a specialist weapon) was a period of minority rule and authoritarian government. Even when the medieval knight was made obsolete (along with his stone castle) by the invention of gunpowder and the appearance of firearms, these new weapons were so expensive and so difficult to use (until 1800) that minority rule and authoritarian government continued even though that government sought to enforce its rule by shifting from mounted knights to professional pikemen and musketeers. But after 1800, guns became cheaper to obtain and easier to use. By 1840 a Colt revolver sold for $27 and a Springfield musket for not much more, and these were about as good weapons as anyone could get at that time. Thus, mass armies of citizens, equipped with these cheap and easily used weapons, began to replace armies of professional soldiers, beginning about 1800 in Europe and even earlier in America. At the same time, democratic government began to replace” authoritarian governments (but chiefly in those areas where the cheap new weapons were available and local standards of living were high enough to allow people to obtain them).
I also posted this excerpt on r/WarCollege. The top comment seems to be erroneous. I think by specialist and amateur weapons, Quigley didn’t mean to say that the specialist weapons were more complex in terms of their manufacturing or their working mechanism but that they required specialist men to use them. A weapon that is more complex than a spear is a rifle but the rifle is much easier to use, yet much more accurate, than a spear for any common guy on the street who doesn’t have the physical strength and training that spears require. Any other more complex weapons, like a supersonic cruise missile, for instance, are practically useless for maintaining internal order and hence don’t factor in when the question of the system of government is asked/answered.