What type of obstacles do historians have to over come to give a straight forward account of past events

First is primary source evidence. A historian must have evidence for a educated guess on the past events. If he has no proof to back his guess up on it is useless.

example : In africa, if one spear is buried underground, you can assume that a war or battle had taked place. As a counterarguement, it can also be the place where weapons were made and one weapon was buried for a reason. If many spears were found, there is a bigger support on if a war had happened. bones is also another one.

Nomads seeking pastures for their flocks found the area good for farming and soon settled there. They found especially fertile soil in Mesopotamia, the area between the Tigris and Euphrates rivers in what is now modern-day Iraq and portions of Iran, Kuwait and Turkey