The Antaran sphere of influence had, at one time, included a colony on an arboreal planet located near the Denobula Triaxa system. The Antaran homeworld, itself, was located "a long way from" from Xantoras.
The Antarans were subjects of several particularly brutal military campaigns that were initiated by the Denobulans that finally ended during the mid-19th century. At the time, the Denobulans demonized the Antarans, turning them into a faceless enemy that fell victim to a series of battle tactics which resulted in the deaths of 20 million Antarans. It was also during this time that Denobulan doctors were accused of alleged war crimes that would have allowed them to "know a great deal about" the Antarans anatomy.
These series of wars established a complicated history with particularly bitter disputes remaining between the two species which fueled prejudices that lasted well into the mid-22nd century. Much of this was due to the fact that neither side had tried to reconcile with the other, and, in fact, it was believed that every Antaran, from childhood on, was taught that Denobulans were enemies to be feared and reviled.
Following an incident at Xantoras in 2153, which forced Enterprise NX-01 Doctor Phlox to treat an injured Antaran named Hudak, a teacher of xenomythology, it was stated by the latter that he was "the first Antaran to even see a Denobulan in six generations."
Hudak's personal feelings towards Phlox were that he'd "rather die than be treated by him." While Phlox – who himself was raised to believe Antarans were "evil," but who chose to raise his children to embrace other cultures – had certain misgivings about treating Hudak.
Though Phlox felt that the bitter history between the two species "wasn't our proudest moment, but we've done our best to put it behind us," he also pointed out to T'Pol that it seemed as if Hudak was "determined to die just to prove a point." With time, Hudak's feelings began to change after reflecting on Phlox's admission that many of his people "have grown more open-minded since the last war, but there are still Denobulans who fear Antarans, even hate them," including his son Mettus, from whom he had grown distant. (ENT: "The Breach")