Man, I love Trotsky...
Terrorism is no good, but armed (and certainly even unarmed) resistance is fairly effective, evident by almost ever US and Russian adventure in the Middle East and Asia. We erroneously conflate resistance with terrorism when there is not always any kind of "terrorism" (a completely abused word -- one who causes terror? Jeez, my daughter frequently commits acts of terrorism!).
Someone shooting a US soldier or contractor is not likely a terrorist, probably just a resistance member (which we now call "enemy combatants," which is generally implied synonymous with "terrorist" without seeming so as text). Shooting civilians is an act of terrorism, and while there are terrorists among enemies of the US government, US contractors (at least) have also committed acts of terror. Somehow, though, we don't have a massive brain attack and suddenly call the US government a terrorist organization just because of that (well, a few people do). Perhaps a double standard. There are both terrorists and resistance fighters in every nation the US government is currently occupying, as well as inside the United States.
(sorry to post something which seems really obvious... not sure that's even a contribution)
I don't see any difficulty in defining terrorism. None at all.
Things like suicide bombers walking into restraunts and blowing innocent people up.