What is a civil war, really?

At one level the answer is obvious: an internal fight for control of a nation. But in the bloody conflicts that split modern states, our policy makers often understand something deeper to be at work. The vengeful slaughter that has ripped apart Bosnia, Rwanda, Syria, and Yemen is most often seen as the armed eruption of ancient and complex hatreds. Afghanistan is embroiled in a nearly impenetrable melee between Pashtuns and smaller ethnic groups, according to this thinking; Iraq is split by a long-suppressed Sunni-Shia feud. The coalitions fighting these wars are seen as motivated by the deepest sort of identity politics, ideologies concerned with group survival and the essence of who we are.