Shortly after the Galactic Empire established a presence on the Stenax homeworld of Stenos, several Stenaxes traveled off-world and worked as mercenaries or performed other duties for little pay while experiencing little interaction with others. This behavior was part of a plot by the Stenaxes to study other species and determine which were most susceptible to conquest. Placing operatives on Hunter's World, Kabray, Delrakkin, Cilpar, and other planets, they learned to use the advanced technologies found on those worlds while awaiting a signal to attack. A few of the Stenaxes lived as traders, secretly evaluating those they dealt with as opponents. None realized what the off-world Stenaxes' true intentions were soon enough to prevent the future massacres.
In 4 ABY, during the month of 40:1 GrS, hundreds of Stenaxes attacked Imperials stationed on their worlds and killed them; they also fought non-military populations, and all non-Stenax inhabitants were soon dead. Not all Stenaxes who lived away from Stenos participated in the attacks, however. The Stenaxes seized deadly technology that the Galactic Empire had brought to Stenos and launched invasions of Kadril, Arda-2, Sooma, Alzar, Tandankin, and other worlds in the Gordian Reach. Millions died in the Stenax Massacres, with a bombardment reducing the planet Kadril to inhospitable rubble. In 5 ABY, the Mon Calamari, in spite of their decision to focus their efforts on defending the emerging government of the New Republic, left Mon Calamari Space in force to bring the Stenaxes' campaign to an end. New Republic forces defeated the Stenaxes at Tharkos in 5 ABY.
The Stenax crews that survived were incarcerated, and orbital monitoring posts were constructed at Stenos to monitor their activities. Following the massacres, the New Republic closely watched the Stenaxes' access to advanced technology, and while the off-world Stenaxes who had not taken part in the attacks were allowed to continue living off of Stenos, New Republic Intelligence observed them from a distance. Historians would compare the massacres to the actions of the Yevetha during the Black Fleet Crisis. Many Stenaxes settled on Belderone and Kulthis, and based on their behavior there, sociologists cautioned that another event similar to the Stenax Massacres could happen.