Since everyone else is doing it, I might as well as present my concept:

2290. [b]The Commonwealth Minutemen[/b] emerged as victors of a bloody war that consumed all of Boston. First, the Prydwen was bought down by a hail of artillery fire, effectively ending the [b]Brotherhood of Steel's[/b] presence in the region. Then, the underground headquarters of [b]The Institute[/b] was destroyed along with the ruins of C.I.T. All while the [b]Railroad[/b] was spared but ultimately bent the knee to the Minutemen. And it was all thanks to a pre-war relic: the [b]Sole Survivor[/b]. 

The Sole Survivor emerged from Vault 111 and found themselves in a whole different world. If it weren't for Preston Garvey and his charity, they would've not survived a day longer in the post-nuclear world. And after helping out a few settlements, Preston promoted them to the rank of general and became second in command. Eventually, with help from newly recruited minutemen, the Commonwealth Minutemen took back their former headquarters, the Castle, and began rebuilding. 

But as tensions between the other factions heightened, rumors surfaced that relations between Preston and the Sole Survivor were declining after retaking the Castle. And things only got worst once the War of the Commonwealth began. After that, arguments between them became more common and known by other minutemen. Then, while fighting the Institute, the Sole Survivor didn't activate the emergency relay and knowingly pressed the detonator. Upon learning that the General essentially murdered the Institute, Preston cursed them for "leaving a black mark on the Minutemen." 

Now, a few years since the destruction of the Institute, critical individuals throughout the region met and reestablished the [b]Commonwealth Provisional Government[/b].