""My boyfriend knows a lot about military history, he owns a few of the books mentioned in this thread. He doesn't read much nowadays though he has read more books in the past. A lot of his military books are about ie. tanks or planes of a specific country in a specific time frame or something like that.""
The first man, presumably, would have some conflicts over resources, then about 10,000 years ago in Pakistan and Iraq the first villages appeared as agriculture began supporting a fixed population. Next walls and forts as surplus agricultural were available, and the first classes who did not work in ag.
Once man became fixed and had something desirable he had to stand and defend it. He would have to fight mobile raiders. In a town artisans arise and military products are produced as being a necessity as much as food production and preparing, storing. A leader/managerial class would evolve - ones who then had intellectual time and resources to run this community.
Virtually all archaeology is going to have defense/offense, religion, and burial as a core. With a professional artisans, some professional soldiers, priests, and leaders man became us. And military was foremost. Reverse entropy, defend or revert to warring pastoral tribes.
You have a walled city and inside professional soldiers, Priests, and rulers as the top of a pyramid supported by an agricultural base. This is the thing which allowed intellectual advancing - the top people were going to be the exceptional ones and they could think, innovate, and direct resources as they were freed from constant labour of producing their needs directly. War was the biggest issue always. Use it or lose it.
If you are interested in what man is, in societies and governing, politics, technology, industry, philosophy, science, architecture, military is the foundation of it all. This need to produce the innovation which defeated the Generals who were always fighting the last war again, and thus losing, drove man forward - where peace typically was the period between when commerce and the ideas learned from war were stored up to be spent on the next burst of fighting.
Paradigm shifts in war are the landmarks in human advancement. Castles in the medieval age everywhere, then cannons in the 1300's and they were obsolete. Henry V and his long bow. The first tanks and aircraft, aircraft carriers, in WWI, and then being the main issue in WWII, you got with the times or lost.
Societies with their politics and philosophy are military - Assyrian, Mongol, Roman, Germanic, Viking, Aztec, Zulu, Shogunate, Persian, each had a huge war industry, one which drove production and innovation - in materials, strategy, governance, and the philosophy of their societies.
Without war we would be digging in the ground with digging sticks and driving goats with some rudimentary oral tradition of superstition and ancestors. War is the imperative which drove us to innovate and industry. The surplus from this gave more and more people freedom from endless toil in agriculture to higher works - including thought. War is man's greatest achievement, unintentionally, but actually. It forces a complex society to form, and then to use that society to innovate and produce the most it can. War destroys an indolent and slothful and static society.
The other side is ethics. In the absence of structured fighting Nobility almost cannot exist. Our ideals of bravery leading us to sacrifice ourselves in fighting for your people. Ethics and military - one of the most fascinating things to think on. Take Nagasaki - it is realistic to say it saved 6 million lives at the cost of 120,000. (includes radiation poisoning deaths) But even if it saved none - it would fascinating in that way too, military ethics holds evil and good in its span. And it is the base of all we are. Manichaeism in effect.