Having written, somewhat hastily, my short post "A note on 'just war'" (see below), I took a look at ch.8 of Hedley Bull's The Anarchical Society (1977), the chapter on "War and International Order," because I recalled his passing reference to Grotius's three just causes for war ("self-defence, the recovery of property and the infliction of punishment": p.198) but couldn't remember offhand what the second and third were.
However, something on the previous page proved more interesting: I had forgotten that Bull has a passage about the decline of interstate war and the rise of civil war. This is a point that is now made all the time but Bull made it roughly 35 years ago: "...the obstacles standing in the way of resort to war between sovereign states have encouraged the tendencies making for war or violence within them. International war, as a determinant of the shape of the international system, has declined in relation to civil war." At the bottom of the same page he refers to "the now circumscribed political role of...interstate war." (So the next time someone in an article refers to the decline or near-disappearance of interstate war, she or he can throw in Bull, The Anarchical Society, p.197, somewhere in a string citation. It conveys a "see-I've-read-the-classics-I-don't-just-crunch-numbers" message, if the hypothetical author wants to convey that.)