After gaining Getting control Control of the former Achaemenid satrapy of Gandhara , including the city of Taxila , Alexander advanced Cause_to_make_progress into Punjab , where he engaged in battle Hostile_encounter against the regional king Porus , whom Alexander defeated Conquering in the Battle Hostile_encounter of the Hydaspes in 326 BC , but was so impressed Change_sentiment by the demeanor with which the king carried Carry_goods himself that he allowed Preventing_or_letting Porus to continue Motion governing Control his own kingdom as a satrap .