Don't Tread on the Bear!

 is a side quest in Fallout: New Vegas.

Quick walkthrough
The exact contents of this quest vary per your actions. It can result in one of four outcomes:
 * The NCR has a robust network of informants. If you continue to work with Mr. House, the NCR will no longer accept your help.
 * The NCR has a robust network of informants. If you continue to work with Yes Man, the NCR will no longer accept your help.
 * Little escapes the eyes of NCR Rangers. If you continue to work with Caesar's Legion, the NCR will brand you a terrorist.
 * The NCR has declared you a terrorist. Wherever the NCR has jurisdiction, you will be shot on sight.

The first three actions are, obviously, the result of aiding an opposing faction. The fourth is a result of gaining general NCR infamy, usually through killing their troops. In the case of opposing faction quests, it will first activate as a warning upon the completion of a certain quest in the series. Completing another quest will fail the mission. You will no longer be able to take on NCR quests once this happens, and any NCR quests active will immediately fail, as well. In the latter two cases, NCR rangers will also be sent to kill you.

Behind the scenes
"Don't Tread on the Bear!" is a reference to the "Gadsden Flag", a historical flag that depicted a coiled rattlesnake over the phrase "Don't Tread on Me."