The last film you watched....

[Paullus Scipio]

Was there anything in Alexander that gave an indication why the Indian elephants retreated? From what I remember, it looked like they were giving the Macedonians a beating, then they retreated, and there was no pursuit. I didn't/don't get it. Seems kind of unlike what I know of combat in those days.

I don't mean to sidetrack the thread.

I'd agree - the movie didn't get what happened across at all....but, the answers are in our sources - Arrian V.17-18; Curtius VIII.14-30.
It should be remembered that elephants are not some sort of ancient"tank" (though often described as such).They are living, intelligent creatures. Their most famous effect was that horses unused to them would not go near them - making them very useful in a strung out line to prevent cavalry attacks. Against infantry, their effect is largely psychological ( as anyone who has seen an elephant charge, even on T.V. can vouch), though physically they take some stopping ! Nor can they just 'steamroll' over formed infantry, but have to resort to short rushes in, then backing out etc. ....and of course, even Alexander had been informed of how they would often, in pain, rage, and fear, as easily trample back on their own side..... as Curtius reports Alexander telling his men;"It is a doubtful kind of strength, and rages more violently against its own men; for it is driven against the enemy by coaxing and command, but against its own men by fear"

In this case, Alexander made careful preparations to deal with the beasts....
The Indian cavalry were not up to the standards of Alexander's elite Macedonian and Persian cavalry, and were driven in, by accident or deliberately sheltering, on the elephants on the flanks. Those in the centre were attacked by the line of light infantry ahead of the phalanx - Agriainians and Thracians - showers of javelins were hurled into the elephants and at their drivers. Nevertheless the trumpeting of the elephants "caused great terror, and their trumpeting not only threw into confusion the horses, animals fearful of everything, but also terrorising men in the ranks.." Curtius VIII.23

Some, getting too close were picked up in the trunks of the beasts and then hurled down to be trampled or passed up to the drivers for slaughter....this combat continued for some time, the light tropps dodging the elephants. The elephants were gradually driven back and in on their own troops, where they caused great destruction. They grew weak from loss of blood, their charges feebler, and as a bull's strength is drained before the kill in the bull-ring, so the hordes of Macedonian Light troops harassed the elephants.
"The Macedonians could deal with these maddened creatures comfortably enough....whereas the unfortunate Indians, in close order, and jammed up as they attemped to get away, found them to be a more dangerous enemy than the Macedonians.." Arrian V.17-18

The elephants began to back away, hooting feebly as elephants in distress do, many of them driverless, and the Agrianes and Thracians, having exhausted their javelins, moved in with axes and kopides(machete like curved swords, like a large Ghurka kukri) with which they had been specially issued. These they used to hamstring the animals and to hack at their trunks and feet.... Porus himself, mounted on the largest elephant, suffered nine wounds in his back and chest, despite armour and lost so much blood he was too weak to throw his javelins. His driver/mahout tried to get the King away to no avail....there was no confrontation between Alexander and Porus.....the movie scene was inspired by the coins Alexander issued later, showing him mounted, rearing up and confronting Porus on his elephant.