Alexander the great, was obliged to go back because

Alexander the great, was obliged to go back because Correct Answer his forces refused to go further

The last few campaigns of Alexander were not one of glory. Most of the Greek soldiers were revolting, they ought to because most of them had not seen home for many years. The initial batch of skilled soldiers who had won swift victories were either killed in battle or they had to be sent home. The Greek soldiers were replaced by soldiers from various nationalities (Persians mainly). As a result conflicts within the troop were common.Alexander was also tired from soul and his dreams were fading. He understood the logic of not moving forward. In the meantime, rumors spread among the Greek forces that there were thousands of soldiers on the other side of Ganges waiting to slay them. Nanda had a huge army, which was many times larger than Alexander’s small forces. He had the dared elephants (Greeks were quite new to the use of Elephants in battle). So, men could not be persuaded to cross the Ganges. Understanding that his dreams would have to wait, Alexander decided to return back. He gave Porus his kingdom and appointed Seleucus Nicator as the regent for India.