History can be spoken with certainty to a certain point in the past. We can say that we generally understood what happened, but we may not have all the information, and some information may be ignored. For example, the biography of John Marshall at http://nobelprize.org/nobel_prizes/peace/laureates/1953/marshall-bio.html shows him to be a very peaceful man. However, if we look at other evidence he created the Marshall Plan.
Another example is Kruschev's so-called secret speech. Many thought that this speech was intended to reach a larger audience. However, the information at http://muse.jhu.edu/login?uri=/journals/history_workshop_journal/v062/62.1rettie.html shows that Kruschev never wanted the speech to be secret, and took special steps so that it wouldn't be.
A third example is the biography of David Ben-Gurion, listed at http://www.jewishvirtuallibrary.org/jsource/biography/ben_gurion.html. It gives the impression of a peaceful leader. But it was later proved that Ben Gurion's men started many of the wars in the middle east.
These sources show how we should be cautious in being certain about history.