How Insurgencies End

[William F. Owen]

Into which of those categories would you place the current conflict in Afghanistan? As you break it down above, it would seem to be regular warfare (more than one state involved) but the tactics seem those most would call "irregular". Regular warfare by irregular means?

Currently, A'Stan is classic irregular warfare. Warfare! NOT War. There is only one kind of War.
Yes, people who have never really thought about tactics do seem to differentiate between regular and irregular. I have never worked out why.

State A invades State B; this is regular warfare by any standard. The Government of State B collapses, but organized resistance continues... is this still regular warfare, or does it become irregular?

It becomes irregular and if a regular party becomes involved it will be both, like Vietnam.
The point really is that eventually the distinction can be irrelevant. WHO fights is very often HOW the fighting is done. In the end the job is still to kill capture and destroy enough to break the enemies will. That will never change.