The Tal Diann is the Romulan Star Command’s internal affairs division, and as such, fills a multi-function role in the Romulan Star Navy.
Many years ago, when the Tal Shiar began to assume a role of major importance in Romulan society and politics, the Romulan navy realized that trusting to the intelligence service’s good graces might prove unwise. Since the Tal Shiar seemed intent on subjecting the military to the same stringent scrutiny as the rest of the populace, and began exercising its sweeping powers of conscription more and more frequently, the military created its own intelligence service, not to gather information on the Empire’s enemies, but to keep tabs on the Tal Shiar. The Tal Diann polices its own, and operates counter-intelligence operations against the Tal Shiar.
In Romulan society, individual Senators can amass a great deal of power. Many rise through the military ranks, and command fleets of their own, which presents certain logistical problems. Aristocratic families placed their sons and daughters in the military, giving them de facto control of certain fleets, which eventually became hereditary prerogatives. Praetors could not necessarily count on the loyalty of these Senatorial Fleets, who sometimes paid more homage to their individual Senator, rather than the Empire as a whole. In an effort to ensure the Empire’s control, in 2268 Praetor Kanetar conferred more political duties upon the Tal Diann, as a fail-safe to the Tal Shiar. Today, the Tal Diann maintains and monitors relations between the Star Command and the various fleets, as well as between fleets themselves.
Lastly, in addition to policing the military, the Tal Diann operates as the military intelligence branch of the Romulan military. Because of the secretive, some might privately say conspiratorial, nature of the Tal Shiar, it might choose to distribute (or not distribute) information to the navy based on what was best for the Tal Shiar, not what was best for the navy. To ensure it received all the intelligence it needed to do its job properly and keep its ships and crews safe, the Tal Diann executes espionage activities against the Empire’s enemies.
The navy recruits Tal Diann agents from among soldiers undergoing basic training. It removes from regular military training those individuals who show promise on certain aptitude tests and send them to an espionage training camp on the world Coravus Prime. They spend approximately one year there studying the ships and military technology of the Star Empire’s enemies, perfecting basic espionage techniques, and receiving political indoctrination. At the end of this year, the navy posts each new agent to the Tal Diann cell where it feels that agent’s talents can best serve it. For example, an agent with a talent for traffic analysis might receive an assignment to a Tal Diann listening post on the edge of the Federation Neutral Zone, while a particularly clever agent from Pallor—home to Senator Kassus—might end up on one of Kassus’ ships, as a junior officer.
The navy designates the head of the Tal Diann as "Chief of Military Intelligence," or CMI. The CMI holds the ranks of admiral in the Romulan navy, and thus stands on equal footing with the other Star Command leaders. The CMI holds a seat on the High Tribunal, much to the consternation of the Tal Shiar. Like so much of Romulan society, with its Byzantine network of connections, the CMI also reports directly to the Praetor.
Several junior officials, each holding the rank of centurion, serve underneath the CMI. These include the Chief of Planetary Forces Intelligence, Chief of Stellar Forces Intelligence, and Chief of Operations. The latter holds the responsibility for planning and executing those operations against the Empire’s enemies it dares not trust to the Tal Shiar. Meanwhile, the agents under the Chief of Planetary Forces Intelligence and Chief of Stellar Forces Intelligence work to ensure the loyalty of soldiers within their respective commands, ferret out traitorous soldiers before the Tal Shiar can, and execute counter-intelligence missions against the Tal Shiar.
Legates monitor communications, have access to personal logs, and keep track of operations aboard ship. Disloyal soldiers can expect to be ferreted out, before the Tal Shiar finds them, thus keeping political ammunition out of their hands. Legates typically receive copies of a centurion’s orders, as well as secondary protocols about which the centurion knows nothing. Thus, the legate can remind a centurion of his duty, should he conveniently "forget," and has a back-up plan in case the centurion fails. Legates also send frequent reports back to Romulus, through fractal-encrypted transmission. Survival-minded centurions always keep this in mind, to ensure glowing reports arrive at Star Command.
Yet this system is not perfect. Occasionally, charismatic senators, admirals, or centurions subvert agents of the Tal Diann. Legates often find themselves far from Romulus, and must depend on the good graces of their assigned Centurion. More than one legate has met with an "accident" during a mission. Through threats and bribery, some Tal Diann agents have been "persuaded" to use their far-reaching powers to provide information on rival ships or fleets, thus aiding the very people they’re supposed to watch.
The Tal Diann is more than the highly visible legates, however. Several junior Tal Diann agents are assigned to each ship, and form the base of the legate’s espionage cell. In addition, recruiting quislings from among a warbird’s rank and file is not difficult—loyal soldiers can always be persuaded to serve the Empire’s goals. These operatives report on activities throughout the ship, and as such, represent a "fifth column" operation on board every ship.