I think it's pretty much Terenas himself who inadvertently sped up the membership of the Cult of the Damned and greatly contributed to the Burning Legion's Third Invasion. I mean because he decided to push to tax for the Internment Camps, it put the Alliance of Lordaeron in a very tight spot:
- Gilneas Leaving, and them building the Greymane Wall
- Arathi Leaving
- High Elves Leaving, which subsequently fueled Othmar Garithos hatred towards the High Elves; and the elves who helped in Warcraft 3 were just random volunteers or were commissioned by the Kirin Tor and Silverhand
- The Barovs' desperation that made them accept and build the bastion for the Cult of the Damned
- Many angry farmers and impoverished people joining the Cult of the Damned
- All of the above, domino effect on Gilneas, Stromgarde, and (formally) Quel'thalas not being there for Lordaeron when Arthas was still scourging the Western Plaguelands (at least they had Kalimdor brigades, it's something)
So, title. Would the Cult of the Damned be just some sort of a small group of humans and elves in a small camp? Would the Kul Tirans have completely genocided the Darkspears in Sen'jin Isles? Would Dar'kan Drathir dabble in the Void instead of helping the Cult of the Damned? Would the Centaurs drive the Taurens to extinction? What do you guys think?
Edit: I'm guessing if not the Plague problem, the Alliance of Lordaeron might have been able to help the Gnomes defend their homeland from the Troggs.