Around the 17th century, the Thazkans came to their planet and started mass killings of the populace. They were eventually driven away when the inhabitants of other planets, realising they would be next, helped the Rentali defeat the Thazkans.
A millennium later, there was a movement by the Thazkans denying that the Rentali holocaust had ever happened. Negative public reaction limited their influence, resulting in history not repeating itself. (PROSE: Denial)