Or Nanclus could simply be bringing promises of Romulan aid (cloaking devices for rescuing Kirk from the heart of the Klingon Empire?) to one side of the upcoming war, or at least promises of neutrality in such a conflict - thereby facilitating the conflict in the first place.
After all, for some reason, the Feds and the Klingons had failed to launch into an open war in the decades following Organia, but also in the decades preceding Organia. A balance of terror probably existed there, then, and the Romulans taking sides (or abstaining) might be the one thing that could upset the balance. Nanclus would promise the Federation certain victory, while some other Romulan would promise the very same thing to the Klingons; war would come; and after the dust settled, Romulans would win.
Remember that in this three-party conspiracy, everybody must have believed that their side would win and the other two would lose - but the cabal could only be held together by everybody claiming that at least one of the others would triumph in addition. A third party would be an utterly vital element here, as it would be rather impossible for Cartwright to convince Chang that the Feds would help start the war so that both sides could win! (Sure, there's that universal fear of soldiers that peace will make them unemployed, but that wouldn't be enough to drive the conspiracy; a hope of victory must have been an ingredient there.)