Explanation:

Alexander marched east to the Hydaspes, where Porus, ruler of the kingdom between the Hydaspes (Jhelum)near Bhera and the Akesines (Chenab) refused to submit to him. The two armies fought the Battle of the Hydaspes River outside the town of Nikaia (near the modern city of Jhelum) and Poros became Alexander's satrap. Alexander's army crossed the Hydraotis and marched east to the Hyphases (Beas).Alexander left forces in India however. In the territory of the Indus, he nominated his officer Peithon as a satrap, a position he would hold for the next ten years until 316 BC, and in the Punjab he left Eudemus in charge of the army, at the side of the satrap Porus and Taxiles. Eudemus became ruler of the Punjab after their death.
When they learned that a vastly superior imperial army of Magadh, Gangaridai and Prasii are waiting for the Greeks, all the generals of Alexander refused to meet them for fear of annihilation. Therefore, Alexander had to return. He crossed the river and ordered to erect giant altars to mark the eastern most extent of his empire thus claiming the territory east of Beas as part of his conquests.