After years of continual besiegement by the trolls, the high elves, with defeat nearing, finally decided to seek the aid of the now-powerful humans of Arathor.
I dunno what defeat at the hands of the trolls looked like, but pretty sure it would've ended with the destruction of Quel'thalas and all its denizens. Furthermore nowhere is it stated the humans joined out of "fear" of being next, but rather as a tactical gain, and learning the arcane arts.
Read it and weep.
Even though the elves had at times treated the humans no better than they had the trolls, Thoradin saw the danger in allowing the elf civilization to fall, and agreed to an alliance. The elves would teach one hundred humans how to wield the arcane arts of magic, while the massive armies of Strom would mobilize and join the war against the trolls.
"Saw the danger of allowing ELF CIVILIZATION TO FALL".
Incorrect. The human armies were at that point very powerful. Re-check your sources, or re-examine the timeline. Humans were the prime ingredient in the downfall of the troll empires.