The Battle of the Hydaspes (or Jhelum) was fought by Alexander the Great in 326 BC against King Porus of the Paurava kingdom on the banks of the river Hydaspes (now known as the Jhelum) in the Punjab near Bhera, thought to be located at the site
of modern-day Mong. The battle resulted in a complete Macedonian victory and the annexation of the Punjab, which lay beyond the far easternmost confines of the already absorbed Persian empire, into the Alexandrian Empire.
Alexander's decision to cross the monsoon-swollen river despite close Indian surveillance, in order to catch Porus' army in the flank, has been referred as one of his "masterpieces". Although victorious, it was also the most costly battle fought by the Macedonians. The resistance put up by King Porus and his men won the respect of Alexander, who asked Porus to become a Macedonian satrap.
The battle is historically significant for opening up India to Greek political (Seleucid, Greco-Bactrian, Indo-Greek) and cultural influences (Greco-Buddhist art), which continued to have an impact for many centuries.
The battle took place on the east bank of the Hydaspes River (now called the Jhelum, a tributary of the Indus) in what is now the Western Punjab. Alexander later founded the city of Nicaea on the site; this city has yet to be discovered. Any attempt to find the ancient battle site is complicated by considerable changes to the landscape over time. For the moment, the most plausible location is just south of the city of Jhelum, where the ancient main road crossed the river and where a Buddhist source mentions a city that may be Nicaea. The identification of the battle site near modern Jalalpur/Haranpur is certainly erroneous, as the river (in ancient times) meandered far from these cities.
After Alexander defeated the last of the Achaemenid Empire's forces under Bessus and Spitamenes in 328 BC, he began a new campaign to further extend his empire towards India in 327 BC. Whilst possessing a much larger army, at the battle, an estimated 40,000 infantry and 5,000 cavalry crossed the river in time to engage the enemy. Depending on the source, Alexander was outnumbered somewhere between 3:1 and 5:1.
The primary Greek column entered via the Khyber Pass, but a smaller force under the personal command of Alexander went through the northern route, taking the fortress of Aornos (modern-day Pir-Sar) along the way—a place of mythological significance to the Greeks as, according to legend, Herakles had failed to occupy it when he campaigned to India.
In early spring of the next year, he combined his forces and allied with Taxiles (also Ambhi), the King of Taxila, against Taxiles' neighbor, the King of Hydaspes.
Alexander had to subdue King Porus in order to keep marching east. To leave such a strong opponent at his flanks would endanger any further exploit. He could also not afford to show any weakness if he wanted to keep the loyalty of the already subdued Indian princes. Porus had to defend his kingdom and chose the perfect spot to check Alexander's advance. Although he lost the battle, he became the most successful recorded opponent of Alexander.