Alexander orders the assassination of Parmenion

Parmenion became Alexander’s second in command throughout the conquest of Persia and commanded the left wing of the army at the battles of Granicus, Issus, and Gaugamela. When Alexander continued eastward after the conquest of the Persian Empire, he left Parmenion in Media to guard his communications. During the campaign, Philotas, Parmenio’s son, was charged with conspiring to murder Alexander, tried, and put to death. Alexander ordered the assassination of Parmenio after Philotas was charged. After Alexander had killed Philotas, the murder of his father was inevitable. In Ecbatana, he controlled the road from the Mediterranean to the East, possessed large sums of money and commanded many troops. Parmenion was too powerful to remain alive, especially since he would be angry when he heard of the execution of his son. Therefore, Alexander accused him of treason and sent an express messenger to Ecbatana, whose duty it was to be there before the news of the death of Philotas reached his father.
The most impressive action of Parmenion's career took place after the battle: he rushed to Damascus (350 kilometers through enemy territory) and seized Darius' treasure. The surprised Persian garrison gave him almost 55 ton gold, a great quantity of silver, 329 female musicians, 306 cooks, 13 pastry chefs, 70 wine waiters, 40 scent makers, and the women who had lived at Darius' court. Small surprise that Parmenion needed 7,000 pack animals to bring the booty to Alexander.

After this raid, Parmenion encouraged Alexander to take a Persian wife. The king followed the generals' advice and started an affair with Barsine.
During the next year, 332, the Macedonians pacified Syria and Palestine. Again, Parmenion had important commands, while his king went to the south to add Egypt to his empire.

To the east
In the summer, the Macedonian army returned to Syria and invaded Mesopotamia and Assyria. On 1 October 331, the decisive battle took place at Gaugamela. Again, the Persians outnumbered the Macedonians. What happened next, is unclear: the Greek and a contemporary Babylonian source contradict each other (the Greeks state that Darius fled, the Babylonians say that he was left alone by his soldiers) (text). However this may be, it is certain that the commander of the Persian right wing, Mazaeus, attacked Parmenion and the Macedonian left. In fact, the cavalry on the Persian extreme outflanked the Macedonians. However, Parmenion was able to keep the fighting spirit of his men high, so that they stood their ground. This enabled Alexander to lead the decisive charge.