After gaining control of the former Achaemenid satrapy of Gandhara, including the city of Taxila, Alexander advanced into Punjab, where he engaged in battle against the regional king Porus, whom Alexander defeated in the Battle of the Hydaspes in 326 BC, but was so impressed by the demeanor with which the king carried himself that he allowed Porus to continue governing his own kingdom as a satrap. Although victorious, Battle of the Hydaspes was also the most costly battle fought by the Macedonians.
East of Porus’ kingdom, near the Ganges River, was the powerful Nanda Empire of Magadha and Gangaridai Empire of Bengal. According to the Greek sources, the Nanda army was five times larger than the Macedonian army. Fearing the prospects of facing the powerful Nanda Empire armies and exhausted by years of campaigning, his army mutinied at the Hyphasis River, refusing to march further east. This river thus marks the easternmost extent of Alexander’s conquests.