For much of human existence, it appears that organized warfare did not happen, because people lived in small groups far apart, and hardly ever got together in big enough numbers for a war.
But as soon as people started farming, they had to fight wars. When you decide to get most of your food from farming instead of hunting and gathering, you've made a commitment to the land where you planted your seeds. You have to keep that land safe until you harvest the food, or your whole group will starve. So people fought to defend their land. The more men you have, the better you can fight. So people began to fight to get more land, so they could feed more people, so they could defend their land. Men did the fighting, and women stayed home to make and cook for more and more babies, so they could grow up and fight too. Not everybody liked this strategy, but the people who fought and made a lot of babies won, and the other people lost and became their slaves, or their serfs.
It's easy to go to a protest and say that war is bad, but a look at some of the wars that were fought during antiquity will show that people generally believe they have good reasons for fighting wars. Hardly anybody thinks killing people is a good idea, unless they can see no other solution for some serious problems of their own.
One reason that people fight wars is that there is no longer enough food where they are, and they have to find more land and food somewhere, or they and their children will starve. This is probably the cause of all those invasions from Siberia south into Iran and China: the Persians, the Parthians, the Huns, the Turks, the Mongols. It is also the cause of the Germans invading the Roman Empire.
Prisoners of War (Romans, 200 AD)
Another reason that people fight wars is that they are afraid of being invaded, and they believe that they can be safer by conquering dangerous enemies on their borders, than by waiting to be attacked. This is probably the cause of the Persian Wars, and the Peloponnesian War, and the expansion of the Roman Empire.
A third reason why people fight wars is that they believe God wants them to convert people to their religion. This is at least one of the reasons for the Arab invasion of the Roman Empire and the Parthian Empire, and it is part of the reason for the Crusades.
Or people fight wars because they have been fighting them, and it is very hard to stop once you get started. You get started fighting people in order to protect your own boundaries, or because you have been invaded, but then suppose you win? There you are with all these soldiers, trained killers with spears and swords and armor. It's hard to tell them you're going to stop paying them now, and please go home quietly. They're liable to take that badly. Much safer to keep on fighting. This is behind the conquests of Alexander the Great, for instance, and maybe the Assyrians (More about victorious armies).
The Persian Wars
The Peloponnesian War
The Punic Wars
The Hundred Years' War
The Wars of the Roses
Thirty Years' War
American Revolutionary War
American Civil War
World War I
World War II
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