God's views on killing

As I understand it, The Sixth Commandment condemns the act of murder but not killing in and of itself, giving exceptions for self defense, defense of another, war, et al.

I've been considering this lately, because I'm not sure that it reflects that a state of war must exist between nations rather than between groups of like minded individuals. The Mafia wars within its ranks, as do tribes in primitive parts of the world, and there are civil wars and undeclared wars between nations. Also, one can have paramilitary actions against another nation, warlike behavior during a period that is otherwise peaceful, and may other things besides.

Here's the question:

Do privately declared wars - whether between families (feuds), businesses, organized crime families, tribes, or other subset that does not reflect a national organization - count as wars in the Biblical sense that the killing within the scope of said war is 'allowable'?

I mean, it's a virtual certainty in a mob war that you are killing in self defense, because sure as rain is wet, the other guy intends to kill you. Also - if you kill him before he sees you or even knows you're there, it's still defensive, because he fully intends to kill you. Furthermore - it is defensive if you kill him even before the war is declared between the families, because he'll surely be gunning for you in a few short hours/days/weeks, right?

So - is a war a war, or does it require a country and an official declaration of war?

/Bart