Originally, and to a large extent still, history was written by the victor and openly told a story flattering to the powerful and those who had won. A successful warrior became a godlike hero.
As rationalism took over in the eighteenth century and onward, truth and accuracy began to be valued over propaganda, especially in the university. In reputable circles today, the lessons of the past are valued only if they are based on truth. If history has something important to teach us, the reasoning goes, it must be accurate history or it is merely teaching us a fiction. False history is an attempt at mind control.
No history can be entirely objective, for we bring to our readings of the past the preoccupations and distortions of our historical moment and our own culture. This is why histories are constantly being rewritten. Nevertheless, it is important to try to get the facts straight so that we can derive the correct lessons from the past. This involves relying heavily on reputable primary sources and constantly reexamining our own biases. It takes humility to write good history. Only history that tells the truth (as best as it can) can guide us accurately to avoiding pitfalls in the future.