How we cite our quotes: (Book.Chapter.Paragraph)
Quote #4
In all the different republicks of antient Greece, to learn his military exercises, was a necessary part of education imposed by the state upon every free citizen. (5.1.12)
Back in the old days, people had to be in the army whether they wanted to or not. You'll still see this nowadays when a country does something called conscription, which means that they randomly draw names from a pool of people living in the country and force them to fight in a war.
Quote #5
The art of war, however, as it is certainly the noblest of all arts, so in the progress of improvement it necessarily becomes one of the most complicated among them. (5.1.14)
The farther history goes, the more complicated war seems to get. Back in the old days, it was a pretty clear-cut affair, where two sides with swords and horses ran at each other. Nowadays, there's a remote control drone on the other side of the world.
Quote #6
Or, secondly, by maintaining and employing a certain number of citizens in the constant practice of military exercises, it may render the trade of a soldier a particular trade, separate and distinct from all others. (5.1.18)
Smith suggests that one reason military combat has become so advanced in the modern age is because "soldier" has become an official career. In the old days, all soldiers were amateurs who spent most of their time working a day job like miner or carpenter. In modern times though, people become soldiers for their whole careers and get a lot better at warfare.