If Alexander had faced a unified Indian empire

That Makedonians got a hard victory over one of the weaker Indian kingdoms when they had the numerical advantage, but that certainly doesn't mean they could've defeated Dhana Nanda's massive, well-trained army.  Take the numbers into consideration:  Alexander's cavalry would've been outnumbered 1:2 (and the disadvantage would've been even greater had I not rounded about 8,000 horse up to 10,000) and his infantry was outnumbered by 170,000 (and that's not even mentioning the 3,000 elephants and 2,000 chariots)!  Put your bias aside and realize how dominating the numbers of this Indian power would've been.  No 40,000-strong force of the time could defeat a 225,000-strong force.

His terrified troops weren't laughing.  After they heard rumors of a 4,000-strong force of elephants across the Hyphases, they began to mutiny and Alexander had to re-distribute armor to make them feel more protected.

Originally posted by Anujkhamar Funilly enough i've never heard of you Lannes, how did i miss your posts?!

Heh, not sure.  I suppose I'm not very memorable.

He himself may have laughed at those elephants but his men certainly weren't. In fact, part of the reason they wanted to turn around was exactly that, being terrified of fighting thousands of elephants. Either way, you really give Alexander too much credit.

They primarily served as archers (the elephants would do the bulk of the actual attacking), though they had large swords for close-quarters combat. 

You should note that Pauravan army at Hyphases wasn't acting in the typical Indian fashion.  The elephant line was oddly defensive in the battle (presumably had to do with the cavalry transfer that took place).

And how many lived in Alexanders huge empire? At least the number India also had. And also, numbers aren't this important when it is just poor peasant farmers, as much of the population was.

Even Persian numbers don't compare with the manpower of the Indians.  At Gaugamela, we could put the Persians at around 90-100,000, and that's counting the 50,000 reserve levy.  The case is nearly the same at Issos.  And if we were to pull our discussion closer to what the original topic was (a unified Indian Army), the odds wouldn't get any better -  the larger Mauryan army is listed at 600,000 infantry, 30,000 cavalry, and 9,000 elephants.

The Nandas had been fighting for years.  Their initial expansion (southwards) saw them defeat many peoples and kingdoms (Ikshvakus, Kurus, Kalingas, etc).  And the Mauryans certainly proved they were capable of conquest from the get go (they would've been using a large part if not all of the former Nandan army.  In fact, Chandragupta is mentioned as 'inheriting' a huge army from the last Nandan king, which he would soon expand).

In this case you're very swift to limit the numbers of Persians, although most researchers agree to 200,000 and only one mentions your figure (can't remember the name right now) but you're equally fast to accept that Indians could field 600,000 armies. Who would feed them, train them, arm them and finally lead such crowd into battle?

An elephand demands 200 kilos of food per day. 4,000 of them would need 800,000 k per day. If they stay pinned at the sane place for over two days they would starve

That number has come more and more into question in mondern times.  John Warry agrees with around 90-100,000, Delbr�ck even puts them much lower than that, and Victor Davis Hanson agrees at around 100,000.  Anyway, the bulk of the army were ill-prepared reserve levy (of which most saw little to no combat).

I've never seen numbers to the contrary for either the Nandan army or the Mauryan army(if you know some, I'd love to see).  While 639,000 may be a stretch, the Mauryan army was nevertheless going to be larger than the Nandan army(the empire grew and Chandragupta made a specific point of recruiting). 

Remember Indian leaders - or at least some of them - at the time of Alexander, didnt exactly liked eachother. If you keep in mind that it was the king of Taxila, Ambhi (sp?) who invited Alexander to support him and defeat Porus, you can imagine it wouldnt be difficult at all for Alexander to repeat it with other Indian leaders. At the time he defeated Porus, i doubt there was even a united Mauryan empire.

Not always!! Remember the example of Rome during Macedonian wars -especially the third - and the stand of greek cities where even in front of the fear of subjection to Rome, not only they didnt united but on the contrary, a few of them allied with Rome in order to destroy the rest.