After two centuries of the Persian invasion, Alexander from Macedonia invaded India. On the eve of his invasion, there were a number of small kingdoms in northwestern India. The leading kings were Ambhi of Taxila, the ruler of Abhisara and Porus who ruled the region between the rivers of Jhelum and Chenab. There were many republican states like Nysa. In short, the northwestern India remained the most disunited part of India and the rulers were fighting with one another. They never come together against common enemy. Yet, it was not easy for Alexander to overcome so many sources of opposition.
Alexander ascended the throne of Macedonia after the death of his father Philip in 334 B.C. He conquered the whole of Persia by defeating Darius III in the battle of Arbela in 330 B.C. He also aimed at further conquest eastwards and wanted to recover the lost Persian Satrapy of India. The writings of Greek authors like Herodotus about the fabulous wealth of India attracted Alexander. Moreover, his interest in geographical enquiry and love of natural history urged him to undertake an invasion of India. He believed that on the eastern side of India there was the continuation of the sea, according the geographical knowledge of his period. So, he thought that by conquering India, he would also conquer the eastern boundary of the world.
In 327 B.C. Alexander crossed the Hindukush Mountains and spent nearly ten months in fighting with the tribes. He crossed the Indus in February 326 B.C. with the help of the bridge of boats. He was warmly received by Ambhi, the ruler of Taxila. From there Alexander sent a message to Porus to submit. But Porus refused and decided to fight against Alexander. Then Alexander marched from Taxila to the banks of the river Hydaspes (Jhelum). On the other side of the river he saw the vast army of Porus. As there were heavy floods in the river, Alexander was not able to cross it. After a few days, he crossed the river and the famous battle of Hydaspes was fought on the plains of Karri. It was a well-contested battle. Although Porus had a strong army, he lost the battle. Alexander was impressed by the courage and heroism of this Indian prince, treated him generously and reinstated him on his throne.
Alexander continued his march as far as the river Beas encountering opposition from the local tribes. He wanted to proceed still further eastwards towards the Gangetic valley. But he could not do so because his soldiers refused to fight. Hardships of prolonged warfare made them tired and they wanted to return home. Alexander could not persuade them and therefore decided to return. He made arrangements to look after his conquered territories in India. He divided the whole territory from the Indus to the Beas into three provinces and put them under his governors. His retreat began in October 326 B.C. and the return journey was not free from ordeals. Many republican tribes attacked his army. Anyhow he managed to reach beyond the Indus. On his way he reached Babylon where he fell seriously ill and died in 323 B.C.
The immediate effect of Alexander's invasion was that it encouraged political unification of north India under the Mauryas. The system of small independent states came to an end. Alexander's invasion had also paved the way for direct contact between India and Greece. The routes opened by him and his naval explorations increased the existing facilities for trade between India and West Asia. However, his aim of annexing the northwestern India to his empire was not fulfilled due his premature death. His authority in the Indus valley was a short-lived one because of the expansion of Mauryan Empire under Chandragupta Maurya.