How we cite our quotes: (Book.Chapter.Paragraph)
Quote #1
The number of those who can go to war, in proportion to the whole number of the people, is necessarily much smaller in a civilized, than in a rude state of society. (5.1.11)
In a civilized society, there are fewer people who can go to war compared to an uncivilized society. That's because people need to stay behind to run farms and make sure the production of the country doesn't crash and burn because everyone is off busy fighting. In a small tribe, the whole group can pick up and travel to another place, so there isn't the same demand for people to stay and work a specific piece of land.
Quote #2
In the agrarian states of antient Greece, a fourth or a fifth part of the whole body of the people considered themselves as soldier, and would sometimes, it is said, take the field. (5.1.11)
In the old days, a large part of the society would consider themselves soldiers. That's because the standards for being a soldier weren't as high. Soldiers nowadays spend their entire adult lives training just for the possibility of going into combat. Back then, it was more like, "Hey, you look pretty tough. Come with us to war."
Quote #3
The experience of preparing the army for the field seems not to have become considerable in any nation, till long after that of maintaining it in the field had devolved entirely upon the sovereign or commonwealth. (5.1.12)
It wasn't until governments created huge military budgets that the modern army was born. This is what allowed people to become soldiers for their entire careers. And naturally, this led to much more skilled soldiers.