There are several theories, which historians love to argue over, as to why Alexander the Great did not conquer India. Some of them are way out in the left field and do not deserve to be poked with a stick. There are a few, however, that bear closer scrutiny.
As Alexander marched his army across the known world at the time, the actual layout of the earth had not yet been fully mapped out. There were “edges” to the world at the time, which was the amount of land that man had actually visited, and put to paper. These edges were considered to be the “end of the earth” and to go farther was to invite death, and the unknown that lay beyond.
At the time, many operated under the assumption that the earth was flat, and one could fall off. While this was beginning to be scientifically tested by great minds, superstition was prevailing. The gods and goddesses, as well as the demons of the underworld, influenced the ancient world immensely.
As Alexander the Great pushed his armies through territory after territory, they would have been met with similar weapons and trappings of war. Much of this changed when they reached the border of India. While the Indian populace was not thought to be as advanced as the Greek world, (to be honest, no one was considered to be as sophisticated as the Greeks by the Greeks), they were met with something that they would not have seen previously. These were elephants. Elephants were used as load bearing animals, as well as to carry men into battle, much as camels, donkeys, and horses were used in many other countries. The Macedonian troops would have encountered camels and donkeys routinely, as caravaners employed them to move heavy loads.
Camels could also go longer distances without water, which made them extremely useful when moving across vast deserts. Elephants, on the other hand, would have been a new animal to many of the Macedonian troops. But, Alexander did not really have the element of surprise when it came to India. There had been some trade with the Macedonians and Indian populace in the past, but there was still an air of mystery when it came to the oriental world. Many thought that India was inhabited by giants. Some of the tribes did have extremely tall inhabitants, with some of the leaders being recorded as being 7 feet tall.
Alexander the Great joined forces with a few of the Indian Kings but was unable to take another by surprise. No amount of feints and surprise movements could budge King Porus. Alexander was finally forced to make the first move, leaving King Porus on the defense. Many historians have debated aspects of the battle for years, as there seems to have been miscommunications going on, and orders being confused as to their meaning. While Alexander was initially able to bring part of India into the Macedonian fold, after his death, it quickly fell back into Indian hands, cutting the Greeks out of India once more.