Because economy within a certain region will be heavily depended on what resources that are available in that region.
Since the Arabian Pennisula do not possess a good climate to produce agricultural products, it's almost impossible for agricultural sector to flourish in this region, and therefore they focused their economy on things such as oil, silk, zaitun, etc.
SO many different battles did not occur in world war 2 your going to have to be a bit specific
mongols were engineers they developed taxes also as they expanded there empire
