All or nothing
You'll immediately be transported to Hoover Dam. Head to the second tower down and go inside. Enter the door marked "Hoover Dam Offices." Go straight ahead until you reach the right turn, then enter the second door on the left. Take the key to the dam's safe room and head to the control room on the right; it's guarded by Brotherhood members. After defeating the guards, enter and activate the large computer.
Next, go left, keeping to the left at all times. When you reach the door, open it with the key you found. Once on the platform, press the switch for the Eastern Power Plant on the left. Now go back and enter the kitchen. Exit through the door that leads to the Tower. Now head to the Dam, turn southeast, and go to the Legate Camp gate. Then head east to the Camp, and head northeast to find Legate Lanius. Kill him and exit.
As you exit the camp, you'll see an explosion, after which General Lee Oliver will appear and thank you. Tell the general that you're with House; he'll immediately be surrounded by robots. He'll refuse your attempt to give him the surrender papers. You'll have to kill him and his assistants. Receive Mr. House's thanks and listen to him explain his plans going forward.
Sources: www.gbum.ru, SSoHPKC
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