system
Given the following post, respond as if you're commenting on Reddit:
human
[LoTR] Who were the guys on the giant elephants and how did the orcs recruit them?
gpt
The War of the Ring was not really a war between the orcs on one side and everyone else on the other. Sauron ruled only Mordor personally, but had a great many allies in the south and east who were intimidated, bribed with promises of power and wealth, or who simply had old feuds with Gondor; some of them may have genuinely believed Sauron's claims to divine status as well. While we don't know the exact numbers, it's likely that there were more humans fighting *for* Sauron at Minas Tirith than against him. This is in addition to the Corsair invasion of southern Gondor and the Easterling siege of Erebor that happened around the same time. Sauron was considered unstoppable not because of his personal power or the power of Mordor alone, but because he had managed to consolidate the sizable majority of Middle-earth into a single coalition intent on destroying Gondor and the elves.

Specifically, the Oliphaunts/mumakil were used by some of the Haradrim from the far south. The Haradrim had a long history of wars and border disputes with Gondor going back to the time of Numenorean hegemony during the Second Age. The Haradrim were also frequent allies of Corsairs of Umbar, who had been hostile towards Gondor ever since Umbar broke off from Gondor during the Kin-strife midway through the Third Age. So it wouldn't have been too difficult for an experienced manipulator like Sauron to convince the Haradrim to take up their old feuds again and invade Gondor alongside powerful allies from Umbar, Mordor, Khand, and Rhun.