This is a good read. Excluding some Kaptaan remarks later tsk tsk.
Some points you might have missed if my learning is correct, most of the history of Alexander invasion of India was written in favor of Alexander the not so great after-all. Alexander had to pay 25 tons of gold to King Ambhika of Takshashila (Taxila) to help defeat Purushottama (Porus) of Paurava Dynasty. the battle was so fearsome that Alexander barely survived the onslaught.
As the Greek Historian Diodorus Siculus mentioned
"Upon this the elephants, applying to good use their prodigious size and strength, killed some of the enemy by trampling under their feet, and crushing their armor and their bones, while upon other they inflicted a terrible death, for they first lifted them aloft with their trunks, which they and twisted round their bodies and then dashed them down with great violence to the ground. Many others they deprived in a moment of life by goring then through and through with their tusks"
Here the inaccuracies starts, why would Alexander return the Kingdom after Paying a hefty sum to King Ambhika and return almost empty handed. Alexander also lost his beloved horse Bucephalus, killed by one of Purushottama's son, it is written as "
known to excel all others for they are immortal. Poseidon gave them to my father Peleus, who in his turn gave them to me". Why would he spare a Kingdom which killed his beloved horse."
There is another theory again put forward by the historians that, his soldiers were tired of constant war which is not true, because the old soldiers were sent back with enough gold and slaves and new soldiers kept coming to fill in.
The exact reason may be, after defeating Purushottama, he realized he can't go any further with this, considering his health and coming Nanda empire, which is even stronger having 200000 foot soldiers, 6000 war elephants, 80000 cavalry, 8000 war chariots.
And to add something to the remarks made, as Ganga worshippers, the rulers who ruled in Pakistan at the time were all Ganga worshippers, the King Purushottam traces his dynasty back to Hastinapur in the Gagetic plains. They lost their land due to continuous floods, and to constant power tussle between the Mahajanapada's.