As George Orwell said in "1984": "Who controls the past controls the future: who controls the present controls the past."
This was made brilliantly clear in Flower's book. She investigates the origins of "forgetting" as practiced in ancient Rome from the earliest writings to the second century C.E. She shows that in the early and mid-republic, families were responsible for policing their members, and those that transgressed against the state, were quickly dealt with by banishment and loss of family connections. This was a drastic measure since the Republic and status revolved around a group of families who controlled the government. During this time, the punishment of disgraced members did not extend to the families or supporters (as opposed to ancient Greece). As a result, the Republic maintained a well-functioning system. The person was erased from family memories, but the family continued unaffected and the person was not erased from the state's memory.
This all changed as the Republic started to feel the demands of Empire. Individuals arose who not only eliminated a person, but also violently attacked the family and supporters of that person. All mentions of the group, from monuments to written texts, were destroyed. In effect, the memory of that person and his (almost always a man) family and supporters were removed from the sphere of remembrance. This was especially true when Marius and Sulla came to power, but it began with the attacks on the Gracchi brothers. The effect of these actions permanently changed the collective memory of the Romans. Worse, succeeding upheavals would again rewrite history. This weakened the cohesiveness of Roman identity (and makes it difficult for modern researchers investigating ancient Rome).
Each of the men who came to prominence violently and rewrote history not only changed the collective memory, but changed the future. Because when the present and past are rewritten, those in power directly affect the future.