I work as a professional historian for a major private college in the US South. Many people would be surprised to know that history is one of the most rewritten of all subjects. This is by necessity, and occurs for several reasons. First, there is ALWAYS going to be more history, since we are always moving forward in time. Second, we are always uncovering new sources that allow us to gain a better grasp on the past. Sometimes, you will see people get very upset over “revisionist” history, but history is constantly being revised. A good example of revisionist history is when the documents of the former Soviet Union became available to Western historians after 1991. These formerly secret documents revealed a lot about the USSR that we had not formerly known. This is revisionist history at its best. Third, history needs to be interpreted in terms of the needs of the present. We must always look at past events in the context of their own times, but the lessons that we learn from those past events can vary from generation to generation.
Next, I would argue that history, as we now know it, is relatively accurate. There will always be pieces of the puzzle we don’t have, however, which is why the PRIMARY job of an historian is to research and evaluate sources. Almost all of graduate school for historians entails how to evaluate sources for their accuracy, and how to then form an educated opinion of what happened. We even have classes on “historiography”, which is the history of history… those who wrote it, the most important opinions on a topic in a field, and so on. A good historian will examine sources from all sides, and try to form as balanced of an opinion as possible. They will also CITE those sources so that their work can be corroborated and checked, and so that other people can form their own independent opinions. They will also EXPLAIN why they came to that conclusion, and why they utilized sources in the ways that they did. In other words, they should inform you of any problems or biases within the sources. Often times, these might not be obvious to a lay person. A good historian WILL have a position on an issue, and they will also make that position plain for all to see and defend it. A good historian will also inform you of other popular competing opinions that merit consideration. After all is said and done, a consensus will usually emerge about the most likely series of events, and that becomes “mainstream” history. Nevertheless, that opinion CAN change with new evidence, though most of the time these changes are subtle and not dramatic re-evaluations of events.