C.E. Evans wrote:
I don't think it was so much a war as it was a falling out between the Romulans and the Klingons at some point after the Khitomer Conference (and perhaps the Tomed Incident too), IMO. The two sides have probably never been friendly with one another and if their territories butted up against one another, they may have been frequently engaged in occasional border clashes. Both Khitomer and Narendra III may have been in territory that the Romulans considered theirs and took steps to remove what they considered uninvited guests (the Klingon colonists on both worlds).
I can't imagine why the Klingons wouldn't declare war on the Romulan Empire after the Khitomer attack, the Klingons would have considered that a dishonorable attack and 4000 dead Klingon colonists would more then enough reason to justify a war with the Romulans. The Klingons went to war with the Federation for far less then that, the Klingons actually would have reason to go to war with Romulans, but they didn't, that seems very out of character for them.
This would be like after the Pearl Harbor attack, the US didn't declare war on Japan, that wouldn't make sense.
Also at one point there was an time when they got along, near the end of TOS there were hints of alliance between the two powers, with Worf saying the Klingons were allies with Romulans before they attacked and the romulans using Klingon war ships in the "Enterprise Incident". So there was an alliance that went south at some point.
I think there is an interesting story here that has never been told.
Plus Martok and Lt. Klag from "A Matter of Honor" have mentioned conflicts with Romulans.