Alexander the great was a king of the Greek kingdom of Macaedon Born in Pella in 356 BC, Alexander succeeded his father, Phillip, to the throne at the age of twenty and by the age of thirty he had created one of the largest of the ancient world, stretching from Greece to Egypt and into present-day pakistan.
In 327 BC  Alexander the great began his foray into Punjab. King Ambhi surrendered the city to Alexander. Many people had fled to a high fortress/rock called  Aornos, he was taken by Alexander by storm after a successful siege. Alexander fought an epic battle against the Indian monarch in 326 BC. After victory, Alexander made an alliance with Porus and appointed him as satrap of his own kingdom. Alexander continued on to conquer all the headwaters of the Indus river.
for the cause of the invasions, in ancient times trade between india and greek was very good and india was very rich in gold , silver and spices. That must have drawn him....

Alexander the great was the king of a greek kingdom macedon. he was anxious to conquer all over the world . i think alexander might have invaded india by the attraction of its vast wealth and richness. moreover, he was interested in conquering the whole world .
so he might have invaded india .
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its will be too lengthy if i will ans about his invasion to india