It's a common thing to say that our history is writen by winers. In modern times, it's a bit more complicated to alter the history considering the technology we have, but thousands of years ago, one country could invade other, conquer it and alter history as they wish to. Future generations will find no evidence of what happened because everything was destroyed. So we are left to believe what books say is the ultimate truth. Of course, not only books are the indicator to check validity of historian fact, you take into the account many other sources together.

What is your opinion on this? How often was history altered and we can't know that?