Many people know about the Eminence Rouge - Cardinal Richelieu, the "power behind the throne" of arguably the most powerful French King, Louis XIV. Lesser known is the fact that the Eminence Rouge had his own Eminence Grise, Francois Leclerc du Tremblay, a Capuchin friar whose role is very little known even to this day, though he was Richelieu's personal representative and diplomat.

Another interesting figure is Johann Friedrich Struensee, the personal physician to King Christian VII of Denmark, who eventually rose to become the de facto ruler of Denmark. What's fascinating about him is that instead of being a self-serving or corrupt power-behind-the-throne, he was instead an Enlightenment-influenced reformer, passing laws to end serfdom, abolish torture, and grant freedom of the press, etc.

I would be remiss if I didn't mention the Carolingians who spent much of their formative years propping up and taking advantage of the Merovingian kings as "Mayors" of the Palace.

Mayor, it is worth noting, derives from the Latin maior, the comparative of magnus and literally means "greater one." In essence they were the "big guys in the house," the house in question being the royal one.

Pippin of Herstal and his son Charles Martel are the most famous pre-royal Carolingians though not the only ones. Both Pippin and Charles ruled Austrasia and at points all of Francia in the name of the Kings and in fact sometimes went for long periods without having a new king crowned. Yet it doesn't seem to have been in their interest at this point to get rid of the Merovingians and make their unofficial power official. It was only with Charles' son Pippin I that the Carolingians decided to take on the royal mantle they had managed for so long.

And its worth noting that the Carolingians weren't the only mayors running the show in the 7th and 8th centuries. There were other rival mayors such as Ebroin in Neustria who were also using the Merovingians as figure heads for the exercise of their own power.

I am, by design, giving few details and painting with a broad brush, but if you want to follow up on this a great place to start would be Ian Wood's The Merovingian Kingdoms

I don’t know if this is “power behind the throne,” but there is a man who had more influence on how we enjoy opera today than any librettist, composer, singer, or musician ever had. Maybe not power behind the throne so much as the power that built his own throne and then pancaked his buttcheeks out on it for years, and then somehow after his death managed to fade into the mists as barely a footnote to operatic history. This man’s name name was Domenico Barbaja, an opportunist, a womanizer, and all around piece-of-work who couldn’t read music. Heck judging from his epistolary output he could barely read words.

As far as I can tell Barbaja did not like opera all that much, he sort of fell into it. He started out as the poor child of dirt-farmers, and came to the big city Milan to make his fortune. First he was a waiter in a coffeeshop, invented his own drink (basically a chocolate cappuccino), and managed to make enough money from that to have essentially his own coffee chain in Milan. But that’s not where the real money was at, hoho no, Barbaja got enough money together to buy the governent contract to run the gambling in La Scala, which is where he made his real fortune. (Opera houses used to have gambling in the lobby, for rich people’s convenience and also to prop up the opera expenses.) After three years he wrangled the contract to run the gambling and the opera at the San Carlo in Naples (the contracts had been newly thrown together) as well as 2 other theaters in the city, and the rest is unsung operatic history.

From his center in Naples he launched the careers of the biggest composers of that age: Donizetti, Bellini, Pacini and Rossini, and many other smaller composers; the singers Giovanni David, Andrea Nozzari, Manuel García, GB Rubini, Benedetta Rosmunda Pisaroni and the legendary Isabella Colbran were all ponies in his operatic stable. He had a real eye for talent: he managed to nab talented people when they were young and unknown and lock them into extremely long contracts in exchange for steady pay. When they were under these contracts he did something then very novel: he farmed them out to other opera houses in other cities, charging money for their services and pocketing it as they were being paid under their contract.

As his power grew he took over more opera houses, including the La Scala where he got his start. So at one point in history all the opera in two major Italian cities (plus Vienna) was controlled by one man, essentially all the major musical centers at that time, as well as every other opera house’s access to the best singers. There is one painting of him and we don’t know his birthday, but the San Carlo you can go see today is the one he personally built when the old one burned down.

Fortunately he is the subject of a lovely recent popular history so you can read all about him: Bel Canto Bully: The Life of the Legendary Opera Impresario Domenico Barbaja by Philip Eisenbeiss, 2013.

Hitler was well known for his voracious interference in any and all affairs within the governance of the Reich - particularly within military affairs after the military disasters of 1942 when he lost all faith and confidence in the competence or loyalty of the predominately aristocratic Prussian general staff. Though what is often overlooked is how Martin Bormann, chief of the Reich's Chancellery and private secretary to Hitler himself - managed access and communication to the Führer. Bormann was constantly at Hitler's side, and with Hitler's preference for verbal orders rather than written communiques; Bormann often conveyed Hitler's orders on his own prerogative.

Bormann's exclusive access to Hitler and his management of his affairs gave him immense power over the delegation of his authority - especially later in the war as Hitler stopped giving public addresses, managing his large powers and delegations appropriately or at all, and his health began to deteriorate Bormann's control over access to Hitler became tantamount to an assumption of Hitler's authority in many ways. Bormann made many enemies and rivals likes Goering, Goebbels, Rosenberg, and Frank - all of which were competing for interest and leverage over Hitler's attention and authority

Not quite 'power behind the Throne', but more 'power behind Cabinet' in 1936-45 Japan.

An interesting aspect of the militarists gaining and maintaining control in Japan was that it was in part entirely constitutional and thus 'legal'. While they did engage in intimidation and violence (assassinations and even coups) against civilian government, an important step in their rise to power came in 1936.

Since the establishment of cabinet government in 1885, the Army Minister had been a vital part of Cabinet government.The Meiji Constitution of 1889 had made the armed forces responsible only to the Emperor, and not the government executive. However, given the nature of the role of Emperor, what this actually meant is the armed forces essentially controlled themselves.

A 1900 law specifically required military ministers to be active serving members, cementing this even further. This was abolished in 1913, however the Hirota government in 1936 reintroduced this measure - meaning that the military could ensure that military ministers were 'friendly' to the militarists.

The key result of this was militarist control of government. Per convention, the military ministers were among the key figures in Cabinet and essential to maintaining Cabinet. From 1936, the militarists could ensure with the threat of resignation, or refusing to nominate ministers, that cabinet governments were go along with their policies, or not oppose them. Ironically, the first Hirota government collapsed in February 1937 for this very reason - the Army Minister Terauchi resigned and the government collapsed.