> My point being that simply saying that all internal conflict is off limits, and all external conflict is within scope is far tooo simplistic

That's not my position. My position is that historically nations wage wars for their own strategic interests. Interests that the nation needs in order to survive and maintain sovereignty. They do not wage wars for "humanitarian reasons". Wars cannot be humanitarian projects because every war is a humanitarian disaster.

Nations need to wisely and parsimoniously choose what their core strategic interests are -- for example maintaining access to some trade route they depend on, or maintaining access to mideast oil, or not having an enemy put an army on its border, etc. Nations have to pick these carefully and then they use military force to fight for those interests. That's how it's usually done by sane countries - those who are not trying to dominate the whole world.

Now, the number of wars that are waged for reasons of strategic self-interest is already a lot of wars. If you add to that "humanitarian reasons" as a legitimate casus belli, what you accomplish is creating even more wars, and that's the last thing you want to normalize.

Moreover, this new casus belli can be hijacked by an elite to wage war that undermines a nation's interests, because how do you really know that enemy X is such a terrible humanitarian disaster that you are justified in bombing their population into oblivion? This boils down to ideology, as the nation committing the atrocities does not believe it is committing atrocities, it believes its actions are justified. And the nation wanting to wage war on them also does not believe it will be committing atrocities but that this action (the war) is justified. So what you have is a war for ideological enforcement.

And as I said before, it has a secondary effect, in that by training a nation to wage wars purely for global ideology enforcement -- something that Europe last did in the crusader period -- you start down a road that always ends in disaster, because

1) a nation has only limited resources for war fighting, and if the resources it does have are expended in the name of global ideological enforcement, then it will not have the resources necessary to effectively defend its own strategic interests. The key blindspot of American interventionists is always that they never recognize limits. Go read some of the neocon journals - any article by the Kagan/Nuland/Bolton crowd will do -- you will search in vain for any description of limits. No realistic estimation of the forces required, and availability of these forces, availability of ammunition, sealift capacity, of how troops will be supplied and for how long. No discussion at all of limits -- only of ideology. It is just assumed that our power is limitless, because global ideological enforcement requires limitless power.

2) the number of wars waged will be huge. Think of all the nations that don't support gay marriage or our definition of "democracy", or the current preferred flavor of women's rights, for example, and so need to be invaded.

3) it will make enemies all across the globe that will, at some point, unite against us -- something that the US is beginning to painfully understand as it suffers a string of military and geopolitical defeats as well as increasing diplomatic isolation.