Explanation:

Nothing in life is absolutely certain (except maybe death and taxes). For EXAMPLE, it might turn out that the World is really just 6000 years old and all the evidence we have to the contrary was deliberately placed there to confuse us. However, as long as the evidence we have stands credited, we go with the evidence we have and not fantasies with no bearing to reality.

Similarly with the historical record, we go with what we have as long as it seems credible. Now there isn't one battle or event in ancient India, that has remotely as many SOURCES as this battle (the battle of Hydapses/Jhelum) does. The five best sources which modern historians rely on are Diodorus Siculus, Arrian, Quintus Curtius, Plutarch and Justin's ABRIDGED version of Pompeius Trogus' work. Most of them lived within a few hundred years after the battle, and they in turn have depended on the works of people who lived at the time including Cleitarchus, who was Alexander's historian, PTOLEMY and Timagenes, who fought in the battle, and the letters of Alexander himself. Original accounts of the battle are also referenced from the works of Onesicritus and Sotion.

The point is that there is a lot of source material for this event. Historians have read and analyzed these works for a very long time, and found a fair amount of mistakes. But no serious historian has ever found evidence in these works to question that: