Chandragupta only rose to power after Alexander left India, taking control of the kingdom of Maghada and expanding from there, creating the Mauryan Empire. He even claimed to have learned from Alexander's feats and mistakes.
The Maghada Alexander was opting to face was an internally weakened, declining state -- hence why Chandragupta had such an easy time rising to power within it. Ruled by the aging, ailing Dhana Nanda, who was referred to by Porus as "merely the son of a barber", which is an Indian proverb for not only someone born into a low caste, but also a weak man. As such he did not have the loyalty of his court nor his people; Maghada was nothing more than house of cards waiting to be blown apart. In this case it was not Alexander, but Chandragupta who did so, a mere three years after Alexander built his twelve pillars and made to leave India.
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