How did Alexander beat King Porus and his war elephants at the Hydaspes?

In May 326 BC, on the banks of the Hydaspes River in what is now the Punjab province of Pakistan, Alexander the Great met an Indian king named Porus. In what was possibly the Macedonians' most costly engagement they won a decisive victory and captured Porus.

The river was the first problem. Porus drew up on the south bank to repel any crossing, and the river was deep and fast enough that an attempt at crossing would probably doom the attacking force. So Alexander used deception. It was reported that an Alexander look-alike held sway in a mock royal tent near the base, while he quietly moved part of his army upstream and crossed in utmost secrecy, using skin floats filled with hay. By chance a storm drowned out the sounds of the crossing.

The battle itself was an elephant fight. Porus, eschewing the usual tradition of Indian kings fighting from a chariot, sat atop his tallest war elephant. The elephants caused heavy losses among the Macedonian foot, but many of their mahouts were struck down by missiles, and the maddened animals trampled many of their own infantry and cavalry.

Then came a surprise. Alexander spared Porus and made him a satrap, effectively reinstating him as ruler. When he asked how he wished to be treated, Porus is said to have replied "Treat me as a king would treat another king". Alexander later stopped at the Hyphasis, when his weary troops refused to advance further.