From Kill Decision by Daniel Suarez. (Fiction but I like the way Oden, a bad ass special ops dude puts it.

Behind every law is the implicit threat of force, and behind ever vote, is the implicit threat of rebellion. That's the bargain that holds a free society together, and no society with a large power imbalance remains free for long.

Democracy only arose when the ability to deploy force was decentralized.

If you go back to the middle ages, the state of the art weapon's system was the armored knight, he cost a fortune to train, feed, and equip, but a mounted armored knight could overpower almost any number of peasants on a battlefield. The distribution of political power in mid-evil society reflected this. Authority was vested in a tiny minority, and the people had no choice but to obey.

Then with the advent of gunpowder, that all changed. Suddenly you didn't need a highly trained specialist warrior to win on the battlefield. All you needed was a warm body who could fire a gun. Anything they could aim at, the could kill, and from that point the edge in warfare went not to highly trained warriors, but to the side that could field the most people, at which point we saw the rise of the nation state, and nationalism as a concept, as a logistical requirement for fielding and ever larger conscripted army, but this changed the political dynamic. The nobles could no longer ignore the demands of their subjects. Those subjects now had to power to kill them, or to refuse to fight in their wars, so kings began to cede more power and authority to representative bodies of the people, like congress, parliaments and so on.