With Our Powers Combined

With Our Powers Combined is a quest in Fallout 4. It occurs after the ending if the player finishes The Nuclear Option & Old Guns and has (or gains) hostile status with the Brotherhood of Steel. Although this is a Minutemen quest, it can be received even if the player finished the main quest by siding with the Brotherhood of Steel.

Quick walkthrough
Build artillery defenses after speaking with Preston. Build an artillery piece and later assign a settler to man it in five different settlements the Sole Survivor owns.

Once that is done, visit the Minutemen radio operator in the Castle and ask him to give the strike order. Prepare for the attack and defeat the enemy.

Behind the scenes
"With Our Powers Combined" could be a reference to the 1990's TV show Captain Planet and the Planeteers. In the show, 5 kids would combine their separate powers to create the superhero Captain Planet.