Interesting topic, though dangerous to deal with without sufficient caffeine in the system.

To my mind, there is a fundamental divide in ways of applying organized violence - though actually it's not so much a divide as a fuzzy grey area.

Conventional (or regular) war is an artificial construct in which force is directed at the avatar of an opposing body politic. In its purest form, 'civilians' and their works are not affected except incidentally, mostly among those unfortunate enough to be located on or near the battlefield. Both sides have essentially abandoned rationality and entrusted the issue to brute strength. The two avatars contend until one side is either completely destroyed or concludes that it can no longer shield the body politic. Since this construct is completely artificial (and in many ways counter-intuitive), it has to be hedged about by the many laws and rules of war to ensure that it remains a non-rational contest of intellectual, spiritual, and material strength, unrelated to the actual issues at stake.

Unconventional (or irregular) war, on the other hand, is the application of organized violence directly to the body politic - or attempts to shield the body politic from such violence. In other words, at least one side is purposefully evading the opponent's avatar to starve, kill, rob, persuade, energize, terrorize, liberate, etc, the 'civilian' sector. Attacks on the opposing avatar are limited to what is necessary to allow access to the body politic.

No war is an unalloyed example of one or the other, of course.

Victor Davis Hanson alert: I will now use questionable and simplistic historical examples to illustrate.

The Hundred Year's War was largely irregular. It consisted mostly of raids, pillaging expeditions, massacres, and the like, designed to enrich one side directly at the expense of the other. On the flip side, each side was also trying to prevent the enemy from inflicting such harm. The key was that any fighting was directly aimed at gaining access to the body politic. Both sides were manned, trained, and equipped for this type of war - the medieval army was a supremely fit tool for irregular warfare in every sense. From time to time, conventional war was resorted to in the form of pitched battles (Agincourt et al), but this was the rare exception.

The American Civil War was originally purely and consciously conventional, but it was won by the north when they began to employ irregular warfare directly against the body politic of the south - and it was the vulnerability of the south to direct attacks on its body politic that in the end ensured there would be an actual surrender rather than a resort to a continuing guerilla campaign.

World War II had aspects of irregular war in ways obvious (partisans, saboteurs) and less obvious (terror bombing) I say less obvious because terror bombing employed the same tools used in regular warfare (bombers, fighters) in very similar ways as those used to attack the opponents avatar. The difference is that it was aimed directly at the body politic, rather than the material strength of the enemy.