A series of wars between the Denobulans and the Antarans, the last of them three centuries before 2153, in which some twenty million Antarans died.
The Denobulan-Antaran wars are described in the Enterprise episode The Breach. The two peoples fought several times, and in the last war, about three hundred years before the episode, Denobulan forces killed around twenty million Antarans. The peoples had no contact afterwards and each brought up its children to despise the other.
When Enterprise NX-01 rescues an injured Antaran named Hudak near Xantoras, he refuses to be treated by a Denobulan doctor. Phlox will not operate without consent, and has to face the prejudice he learned from his grandmother.
Phlox tells Hudak that he raised his own children to reject that hatred, though his son Mettus took it up all the same. Hudak finally agrees to the treatment, and the two part on better terms.