With Our Powers Combined

With Our Powers Combined is a Minutemen main quest in Fallout 4. It occurs after the ending if the player finishes The Nuclear Option and 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.

Detailed 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.

Watch from the Castle, as the Prydwen is destroyed, and then prepare to fight the Vertibirds that they send. The best strategy is to snipe the Knights within the Vertibirds, and bring as many down as you can, before they drop down and you continue the battle on ground.

Bugs

 * The quest may halt at 90%, with the last Vertibird never arriving and the residents of the Castle resuming their ordinary behavior. This can be resolved by finding the last Vertibird around the Boston Airport and destroying it, triggering 100% completion.
 * The quest may stall at 90%, with the last Vertibird hanging in midair without crashing. Fixed by reloading a previous save.

Behind the scenes
This quest's name is most likely a pop culture reference to the children's TV show Captain Planet and the Planeteers, in which the line "by our powers combined..." is the show's main catchphrase.