The Wookiee [species]' home planet of Kashyyyk was the site of a massacre perpetrated by the Galactic Empire sometime prior to the Battle of Endor. This occurrence nearly led Mon Mothma, the Chancellor of the Empire's opposition, the Alliance to Restore the Republic, to believe diplomacy was futile and that she should begin training as a fighter pilot. Mothma referenced the Kashyyyk massacre to Leia Organa as the Alliance prepared to engage the Empire's DS-2 Death Star II Mobile Battle Station battle station, describing the impact that event had on her at that time.
Tom Angleberger mentioned the Kashyyyk massacre in his retelling of Star Wars: Episode VI Return of the Jedi, titled Return of the Jedi: Beware the Power of the Dark Side!.