Only if hatred is mutual.
You know declaring hatred is declaring everyone and everything to do with their realm to be Evil Incarnate, yeah? Completely savage, ignoble, daimon-worshipping, inhuman, kicking puppies and babies, evil nose-picking bastards! And it doesn't have to be until death. They could continue to remain at hatred without killing each other with on and off again active wars/campaigns. In fact, I'd find that a neat concept. A constant hot-and-cold/active-inactive hatred between realms. The old rivalries for instances, Sirion-Perdan, Yssaria-Caligus, Enweil-Riombara, D'Hara-Luria etc.