www.alphadictionary.com

internecine

in·ter·nec·ine adj.

1. Of or relating to struggle within a nation, organization, or group.
2. Mutually destructive; ruinous or fatal to both sides.
3. Characterized by bloodshed or carnage.

[Latin internecīnus, destructive, variant of internecīvus, from internecāre, to slaughter : inter-, intensive pref.; see inter– + nex, nec-, death.]

Sitran Apoclima

I don't remember ever seeing "internecine" without its journalistic tagalong "warfare".

Johnson was not taken to task for this error. On the contrary, his dictionary was so popular and considered so authoritative that this error became widely adopted as correct usage. The error was further compounded when internecine acquired the sense “relating to internal struggle.”

That's an interesting note in the link you provided, Sitran. Thus mutually destructive antagonism within the city (or school) could more properly be "intranecine" and aggression with clear outsiders as "extranecine". Unless one group really, really wanted the others eliminated at all costs, which becomes a teen video game or "reality" program along the lines of "XtremeNecine".
-gailr

It's a real pity that the first meaning given by Apo, supra, and which is based on an erroneous reading of the «inter-» morpheme, dominates the usage of this word completely, so that journalists who find «internal» too pedestrian use this word instead. Warfare is almost always internecine, whether or not it occurs within the confines of a single state or a single people....

Henri