History is written by humans, and is based upon their perception of the events. Perception is always subjective. Each person's perception is different. Even eyewitnesses that were actually present at any given event, will give you very different accounts of it. To illustrate this point, I recommend that you watch the movie called Vantage Point, if you haven't already seen it. To make things worse, people who write history were not actually there to witness most of the events that they describe. So, they are forced to come up with a way to interpret a number of facts, documents perhaps, which can probably be interpreted in more than one way. Of course, the way they choose to put it will depend on their own personal opinions, and they also may be influenced by their country's official policy. The more corrupt the country is, the more they will twist history to their convenience.
Lastly, when it comes to ancient history, I think that writing it is closer to fantasising than to an actual science. There is simply no way to accurately tell what happened thousands of years ago.
