internecine: Marked by slaughter, especially mutually destructive slaughter; battle within a group.
According to the interesting word history in Merriam-Webster's Collegiate Dictionary, Deluxe Edition, the "mutually destructive" sense of internecine came about because Samuel Johnson misinterpreted the word's etymology when he put together his dictionary. Inter- usually indicates "between" (e.g., intermural, interstate, interlibrary loan). In this case, though, inter- indicated the completion of an action.
To quote directly: "In Latin, the verb necare meant 'to kill' and the verb internecare meant 'to kill without exception, to massacre.'" (p. 964)
Johnson, though, put "to kill" together with "between" and came up with mutually destructive violence. His definition was preferred, though, by many. These days, internecine is more often used hyperbolically, meaning a pitched (though often bloodless) battle between two factions of a single group.
The clash of Tea Party Republicans and rational Republicans might be seen as an internecine battle, for example.