Defiance Has Fallen

Defiance Has Fallen is a main quest in Fallout 76. It is part of the Brotherhood of Steel quest line.

Quick walkthrough
To start the quest, go to Abbie's bunker which can be found in the Mire region of Appalachia, and read the "Fort Defiance" entry on her terminal.

Abbie's bunker
Travel to Abbie's bunker and look for her terminal which is located in her bedroom. Once there, read the entry titled "Fort Defiance."

Camp Venture
Next, you will need to travel to Camp Venture which is located directly east from the National Isolated Radio Array. Be prepared to clear the area of creatures. Once you have cleared the area, you will need to find the secure storage key in the bunkhouse. It will be locked in a locker that can be picked. After you obtained the key, find your way to the V.E.S.T. building on the lower side of the camp. in the basement, there is an orange door. Unlock it with the key and look inside for a note on a table called Command Center password. Next, you will need to find the Command center building which is guarded by a terminal. Once inside look for the Letter to Tommy which is located on a table.

Fort Defiance
Fort Defiance is located on the edge of the Cranberry Bog and Savage Divide inside the Allegheny Asylum building. Be ready to fight more scorched.