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.