All or Nothing

 is a quest in Fallout: New Vegas.

Quick walkthrough
You will spawn at Hoover Dam in the middle of a battle. There will be several factions that you will have to deal with to get to the door, but you will have the help of several Securitrons. After getting inside and installing the override chip, you will need to take a fairly short walk and turn on the power to turn on the Securitron Army. After that, you must exit the Dam and head east towards the Legate's Camp and kill the Legate and all enemies from Caesar's Legion. (It is also possible to talk them into leaving the dam, using a speech skill of 100). Once you have killed all enemies in the Legate's Camp, you must head back west where you came from and talk to General Oliver and deal with him and the NCR. You may be able to avoid killing Oliver and his men if you pass a Speech challenge (100 Speech), although it doesn't make a difference because the quest ends the game. After dealing with Oliver, talk to Mr. House and end the quest.

Detailed walkthrough
You will spawn at Hoover Dam were immediately NCR Rangers will be fighting the Great Khans and Ceaser's Legion. It is great to have some apparel of these factions so you can slip on by, although your campanions might get caught up in the fight. The tower you must enter, will be the third one on your right. Once you have uploaded the overide, head back up to the surface to defeat the legion. There will be several factions that you will have to deal with to get to the door, but you will have the help of several Securitrons. After getting inside and installing the override chip, you will need to take a fairly short walk and turn on the power to turn on the Securitron Army. After that, you must exit the Dam and head east towards the Legate's Camp and kill the Legate and all enemies from Caesar's Legion. (It is also possible to talk them into leaving the dam, using a speech skill of 100). Once you have killed all enemies in the Legate's Camp, you must head back west where you came from and talk to General Oliver and deal with him and the NCR. You may be able to avoid killing Oliver and his men if you pass a Speech challenge (100 Speech), although it doesn't make a difference because the quest ends the game. After dealing with Oliver, talk to Mr. House and end the quest.