For our background, I tried to draw a parallel between how the 47th started in real life and transitioned into a Roman based guild and how we would do so in the Fallout universe.
The 47th Armored Cavalry Division, stationed at Fort (Wherever) and currently on assignment at (wherever) is under the command of Colonel Maximillian Lawrence. After the war, Lawrence maintains discipline and morale for a short while before the 47th realizes that the world has been destroyed and they are, as far as they initially know, the last survivors of the human race. As a result, morale crumbles and the 47th starts falling apart. Drawing on his near obsession with Ancient Rome, Lawrence adopts the name Labenius and the rank of Legate and reforms the 47th into the 47th Legion, and sets off on a mission to rebuild the wasteland in the image of Rome.
At some point after that, Labenius starts to turn more and more despotic, culminating in increasingly questionable orders, ultimately leading most of the Legion to refuse them (like destroying a neutral settlement for their resources) , so Labenius orders a decimation to keep control of his rebelling officers. This is the turning point, and the officers stage a coup and assassinate him However, they realize the charismatic effect that Labenius has on the men, and decide to keep up a ruse where Labenius is still alive and in command, all the while Cavadus is really the one in charge.