While I heartily agree that the DoD is a great place to look, I respectfully submit that the DoD is not a human agent, but a large bureaucracy whose nominal director—the Secretary of Defense—is a political appointee. For my part, I doubt that Lloyd Austin is the true executive of the Executive Branch of the U.S. government.

The question prompted me to ponder notable behind the scenes executives in history. For most of history, power resided in warlords and generals. The Roman Republic was dissolved when the consul Octavian received (from an intimidated Senate) the augmented power of Imperium, or command of the army. For all of his immense power, many suspected that his wife Livia wielded vast influence over his decision-making.

Likewise, Napoleon Bonaparte rose to power in the First Republic of France by virtue of being a military commander. Like Livia, Napoleon's wife Joséphine is thought to have wielded great influence over her husband.

Gaius Julius Callistus was a "notoriously powerful" freed slave during the reigns of Caligula and Claudius. Under Claudius he was suspected of being the true holder of executive power.

Lucius Aelius Sejanus was a formidably powerful Prefect of the Praetorian Guard during the reign of Tiberius, and was suspected of calling many of the shots.

Cardinal Richelieu was suspected of being more or less in charge during the early reign of King Louis XIII.

Madame de Pompadour, the Official Chief Mistress of King Louis XV, was thought to wield great influence over the king.

During the fateful July Crisis of 1914 following the assassination of the Austrian Archduke Franz Ferdinand in Sarajevo, it seems that true executive power resided in Conrad von Hötzendorf, Chief of the Austro-Hungarian General Staff. While the old Emperor Franz Josef was trying to enjoy his summer at his hunting lodge in Bad Ischl, Hötzendorf and other hotheads at court in Vienna pressed for war with Serbia, even if it meant war with Russia.