All or Nothing

 is a final 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.

NOTE: An easy way to defeat the Legate is to sneak attack him using any type of sniper rifle. This will cause his minions to attack you and completely bypass the second wave of soldiers. Kill enough of them and you'll receive the quest update.

Detailed walkthrough
You will spawn at Hoover Dam where immediately NCR Rangers will be fighting the Great Khans and Caesar's Legion. It is great to have some apparel of these factions so you can slip on by, although your companions 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 override, head left until you find the exit door, where just outside there will be the electrical switch to turn on the power to turn on the Securitron Army. Head back in and go back to the dam where only Legion and Securitons remain, and head east towards Legate's camp. Securitrons will be helping you fight your way to the camp, although the easiest method is with Legion apparel. Once at the Legate's Camp, you must either kill the Legate and all enemies from Caesar's Legion or convince the Legate to surrender with a speech challenge of 100. Once your done with Legate's Camp, you must head back towards the gate where you must deal with General Lee Oliver and the NCR army. You can either kill Oliver and his men or convince him to back down with a Speech challenge of 100. After dealing with Oliver, Mr. House will roll up and end the quest and game.

http://www.youtube.com/watch?v=ZmQK-4x82ZI << A walkthrough on how to defeat The Legate Lanius on "All or Nothing"