In ancient times, kings had no mercy in them; they only thought of getting power and more lands in control of them. War was a medium where they could fulfill their only ambition - to get power and land. Wars usually used to take place between two states or cities. In addition of getting power and land, they also got the treasures of the captive land. Winning a war against a country was equivalent to exploiting the country of its riches(I personally don't think that it was a very nice act); so, it was quite necessary for the kings, dukes and nobles.