In the Star Trek movie 'The Undiscovered Country', what role did Ambassador Nanclus play in the conspiracy?
Did he act to prevent the Klingons and the Federation from forming an alliance?
He was at the least a conspirator
In that year, he was involved in a conspiracy to sabotage peace talks between the Federation and the Klingon Empire. While in a meeting with the Federation President after the arrest of James Kirk and Leonard McCoy, Nanclus lent his support to "Operation Retrieve," the proposal to infiltrate Klingon space and rescue the hostages, put forth by Nanclus' fellow conspirator, Colonel West. Later, Nanclus attended the Khitomer Conference. After the crew of the USS Enterprise-A learned of his involvement in the conspiracy, and successfully prevented the assassination of the Federation President at the conference, he was detained by Starfleet. (Star Trek VI: The Undiscovered Country)
Anywhere that you look on Memory Alpha they all say that he was a co-conspirator to undermine peace talks, so whether or not he was involved in the rest of what was going on, I am sure that he was at the least trying to undermine the peace talks and this gave him some ammunition
David A. McIntee once proposed that he may have been the initiator of the conspiracy in a Star Trek Magazine article
Author David A. McIntee once proposed that Nanclus was actually the initiator of the conspiracy, having secretly put the idea that the Federation were the true threat to the Klingon Empire into General Chang's mind, due to Romulan concerns that the Klingons and Federation were about to form a pact. McIntee went on to characterize Nanclus as "almost certainly the true villain of the movie, and probably the most forgotten of the Trek villains." (Star Trek Magazine issue 149, p. 64)
Obviously the Romulans didn't want the Klingons and the Federation teaming up, because the Romulans couldn't possibly overpower them in tandem, which means that the Romulans could never expand into Federation or Klingon space.