Don't Tread on the Bear!

Don't Tread on the Bear! is a side quest in Fallout: New Vegas.

Detailed walkthrough
The exact contents of this quest vary per actions. It can result in one of five outcomes:
 * The NCR has a robust network of informants. Continued work with Mr. House makes the NCR neglect help.
 * The NCR has a robust network of informants. Same as above, but when working with Yes Man.
 * Little escapes the eyes of NCR Rangers. Continued to work with Caesar's Legion gives the NCR's brand of a terrorist.
 * The NCR has declared you a terrorist. Wherever the NCR has jurisdiction, be ready for being shot to be killed.
 * The NCR knows that you are working against its interests and will no longer accept your help.

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 or launching the missile against Long 15. 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, the entire faction will be enemies with you giving a preparation to die. The NCR-based ending is no longer possible to achieve.

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."