You've read this adjective many times (sometimes preceded by words like "deadly" and almost always followed by nouns like "warfare", "struggle" or "conflict"). But what does it actually mean? Most commonly it's used to mean "deadly and internal".
Some people, however, insist that its true meaning is simply "internal" (the "inter" prefix is surely the giveaway here). However, history is not on their side.
In its first recorded use in English, in 1663, "internecine" meant "fought to the death". About a hundred years later, when he was writing his famous dictionary, Samuel Johnson defined the word as "endeavoring mutual destruction". He took as his cue the prefix, meaning "between, among". But according to the American Heritage Dictionary of the English Language, he misunderstood: "inter" was actually being used in its less common guise as an intensifier. Johnson was such a compelling authority, that his new meaning was widely accepted, and gradually shifted over the years to become "relating to internal struggle". That's the restricted meaning some people still insist on.
But meanings are given by accepted usage, not by personal preference. Today, "internecine" is most widely used to mean internal and destructive, and the Oxford English Dictionary, for one, offers that definition. On that basis, the US Civil War was an internecine struggle, but WWII wasn't - it was merely destructive. We'll stick our necks out, in that case, and assert that to talk about deadly or savage internecine warfare is to flirt with redundancy.