In 2267 Starfleet Intelligence Romulans engaged the USS Enterprise using Klingon designed D7-class battlecrusiers; (TOS: "The Enterprise Incident".
In 2364 Worf condemned the Romulans responsible for the Khitomer Massacre of 2346 as "without honor" saying they "killed my parents in an attack on Khitomer at a time when they were supposed to be our allies." (TNG: "The Neutral Zone")
In 2367 Starfleet Commander William Riker reacted to the suggestion that Klingons and Romulans were working together by saying "A new Klingon alliance with the Romlans?" By saying "new" he implied that there had been an "old" one at some point. (TNG: "Reunion")
- While the existence of a Romulan-Klingon Alliance was long accepted by fandom the canonical justification for it is less than slight. Beyond the scant and implicitly contradictory details above the existence of an official Romulan-Klingon Alliance has never been explicitly stated. The stated reason for the Romulan use of D7-class battlecrusiers was the single line of dialogue "Intelligence reports Romulans now using Klingon design" with no justification of this and no use of the word "alliance".
- Stephen E Whitfield's book The Making of Star Trek (which finished during the early part of the third season of TOS) stated that the TOS production team intended to have the Klingons and Romulans form an alliance against the Federation at some point during the series. This ultimately never came to pass. Later fan speculation suggested that such an alliance had taken place and that in return for the Klingon ships the Romulans traded to the Klingons their cloaking device technology. (An explanation for Klingon ships being seen to routinely use such devices from 2285 onwards (Star Trek III: The Search for Spock)). However, there has never been any onscreen evidence to support this logical idea. It is possible that it was the widespread fan acceptance of the existence of such an alliance that led to the latter two references above, them being written as indirect consequences of that acceptance.
- Episodes of later Star Trek series provide additional information which further complicates any notion of such an alliance. For example (DS9: "Blood Oath" mentions a Klingon victory over the Romulans in the Battle of Klach D'Kel Brakt in 2271 a date that sits uncomfortably between 2267 and 2346 two years which are reasonable speculative beginning and end points for an alliance given the scarcity of canoncial data. Equally TNG: "Reunion") set in 2367 states that the Klingons and Romulans have been "blood enemies" for 75 years prior to the episode (so since 2292) a period of time which suggests either that Worf's statement in (TNG: "The Neutral Zone)" is factually incorrect (or at best confused) or that the Klingons and Romulans were 'blood enemies' and 'allies' simultaneously.
- While the existence of a Romulan-Klingon Alliance has been long accepted within fandom the concept is far from canonically provable and the very term itself may perhaps more properly to be considered fanon.