How were the first war elephants trained and tamed?

Yeah, that's a pretty big mystery.

Now it's possible to capture wild Elephants by pure manpower and “break them in”, but it's risky AF and the beasts have this irritating habit of dying of grief if you kill off their herd-mates and enslave them. For this reason (among others), the noble ancestors used to accord near-human intelligence to Elephants.

A man-eating tiger chowing down on half a village or a spooked horse bashing in the brains of a groom could be ignored, but a man-killing Elephant had to be kept under observation. They knew what they were doing.

Or that was how the thinking went.

Anyway…

So the traditional method of training Wild Elephants was to have semi-domesticated herds on “Reserve Forests” continually observed by Forest Officers; no one went around playing with Dumbo in the middle of Dandarkaranya back then unless they were either crazy or suicidal. When need arose, they'd drive these semi-domesticated beasts into enclosures using other Domesticated Elephants as bait/ coercion. Using force was discouraged; the texts were clear- NOTHING short of Military might- disciplined Infantry preferably- could bring down a charging Bull Tusker, the ones you really had to keep an eye on. I suppose some variant of this gradual domestication of local herds was used to build up enough camadiere between man and beast, in order to raise the first truly “domesticated” herds. Zahir Dehlvi wrote about just how intelligent these creatures could be; Maula Baksh- who was the Mughal 'Emperor's' Royal Elephant from the 1790s to the 1850s- was apparently a murderous animal all had to be wary of, except for when children were involved or the Mughal puppet had to ride him about town or to War.

Most of these captured “wild” elephants were usually released, both to facilitate acquaintance with humans among the local Elephant population as well as to provide ready breeding stock. Naturally the training grounds were usually adjacent to the forests themselves.

Now modern knowledge states that it's impossible- or close enough for practical matters- to breed Elephants in captivity- however the Classical Elephant Trainers did manage to pull this regularly enough. I'm not sure though whether this was enough to maintain sustainable populations in more urban/ semi-urban settings.