Instead, while under false arrest and retreating from the Macedonians, Darius was killed by one of his subjects.
Because the battle at Gaugamela marked the turning point in the battle between the Macedonians and the Achaemenids, it is clear that if Darius was to have been able to defeat Alexander and his troops, he would have needed to do so before the battle at Gaugamela. Therefore, it is important to look at the opportunities that Darius had to attack Alexander and his troops prior to that battle. Looking at those opportunities, it becomes clear that Darius' best chance to defeat Alexander's army would have been to attack Alexander before he had the chance to gain the support of the Greek city-states. To do that in the most successful manner, Darius would have needed to attack the armies of Parmenion and Attalus. This would have permitted Darius to defeat Alexander before Alexander could have obtained complete control of Philip II's army. However, defeating those armies would only have been a successful battle tactic if Darius had the type of character that would have allowed him to gain the support of the Greek city-states that eventually supported Alexander.
Many of the available sources seem to suggest that Darius was a man of astounding character. Unlike many rulers of antiquity, Darius prided himself on being one with the common man. In fact, Darius went to his coronation dressed like a commoner. On some level this was a characteristic shared by Alexander. For example, Alexander made certain to personally participate in his battles and fraternized with his troops. However, Alexander also took great pains to separate himself from the common man. Alexander actively encouraged the rumors of his divinity, hoping that his troops, and, more importantly, his enemies would see him as somehow above death.
In addition, it appears that Darius had the type of character required to inspire loyalty from his troops, even when those troops were composed of conquered people. Darius "was loyal to those who supported him. He felt responsible for the well-being of the troops under his command, even if they hailed from alien nations and practiced customs which were culpable to his Persian courtiers." Therefore, it seems very likely that Darius would have been able to garner the support of the Greek city-states if he had taken an offensive position when Alexander first appeared to be a threat to the Persian Empire.
However, a further investigation of Darius' personality reveals a central weakness that greatly reduces the likelihood that Darius ever would have been able to mount a successful campaign against Alexander. It is undeniable that Alexander was brilliant in battle and more than one person has suggested that his brilliance was due partially to his arrogance and his belief that he could not be defeated. Darius did not share this same arrogance. On the contrary, Darius may have been too accommodating and willing to compromise:
Before Gaugamela he made three peace offerings to Alexander. In the first one he addresses Alexander as "Alexander" and himself as "His Majesty." In the third one he is virtually down on his hands and knees. Prior to the final battle Darius in prayer expresses his hopes that after him Persia will be ruled by his "merciful victor."
While this type of accommodation may have marked Darius as a good politician, it did nothing to protect him or his troops from Alexander's eventual onslaught. Instead, in retrospect, it demonstrates that Darius did not have a firm understanding of the enemy that he was facing. Darius seemed to approach the idea of battle with Alexander from the position that both leaders would desire peace. However, had he made a more careful study of Alexander's military advances, he would have come to the conclusion that Alexander was not interested in offers of peaceful coexistence. Alexander was interested in domination, not accommodation. Therefore, Darius should have made it clear that he was not willing to make concessions to Alexander. By demonstrating his willingness to come to a compromise, Darius demonstrated that he did not believe that he would be able to conquer Alexander's troops in battle. Obviously, by the time of the battle of Gaugamela, this may have been true. However, if Darius had gone on the offensive before that time it is possible that he may have been able to gather a larger offensive force than the one commanded by Alexander, which may have been sufficient to assure him of success when they eventually met in battle.
Regardless of the size of his army, one of the aspects about Darius that makes it less likely that he would ever have been capable of defeating Alexander is that some historians characterize him as somewhat indecisive. For example, the Greek historian Arrian indicated that "Darius was always ready to believe what he found most agreeable to believe. He accepted any council that told him what he like to hear." Furthermore, Arrian describes Darius as "feeble and incompetent in military matters." Obviously, if the real Darius matched Arrian's description of him, it is unlikely that Darius would have been able to capture the support of the Greek city-states by moving on the armies of Parmenion and Attalus. In fact, if Darius was actually a feeble and incompetent military leader, he may actually have hastened his decline by opening him up to attacks that would have weakened his army before he ever had occasion to meet Alexander in the field.
However, it does not seem very likely that Arrian's accounts of Darius' personality are accurate. After all, Darius did not hesitate to attempt to recapture the Nile valley for the Persian Empire. Furthermore, Darius managed to hold his own against Alexander, making the outcome of their dispute appear uncertain. The mere fact that Darius was not immediately and resoundingly defeated by Alexander supports the notion that Darius was a capable military leader. Few question that Alexander was a brilliant battle engineer, and to be able to face him in battle, Darius must have been somewhat brilliant as well.
The historian Diodorus believed that Darius was a brilliant military strategist. According to Diodorus, Darius' "overall policy to handle the Macedonian invasion was sound and realistic." Furthermore, Diodorus believes that Alexander's victory was not due to Darius' failures as a leader, but to mere circumstance. He believed that Darius "should have been successful in defending Persia, were it not that his foremost commander Memnon suddenly died in the winter of 334/333 BC." In fact, Diodorus believed that Memnon's death changed the nature of the war and was the key to Alexander's victory.
Diodorus backs up his confidence in Darius' military leadership ability with facts about Darius' prior success in military engagements. In fact, according to Diodorus, Darius was selected as the king because of his bravery in war.
Darius' selection for the throne was based on his known bravery, in which quality he surpassed the other Persians. Once when king Artaxerxes [III Ochus] was campaigning against the Cadusians, one of them with a wide reputation for strength and courage challenged a volunteer among the Persians to fight in single combat with him. No other dared accept, but Darius alone entered the contest and slew the challenger, being honored in consequence by the king with rich gifts, while among the Persians he was conceded the first place in prowess. It was because of this prowess that he was thought worthy to take over the kingship.
As a military commander, Darius entered into a one-on-one battle with a Cadusian warrior champion, which led to the return of the Cadusians to the Persian Empire. This act was significant because the Cadusians were reputedly tremendously able and loyal warriors. Therefore, the fact that Darius alone was able to bring them under Persian control indicates that there was a strong likelihood that early military victories against the armies of Parmenion and Attalus would have given Darius the leverage to ally with the Greek city-states.
In fact, history supports the portrait that Diodorus paints of Darius, while not actually lending support to the histories given by historians like Arrian. After all, Darius was able to successfully reconquer Egypt, a task which had eluded the prior king. Furthermore, the manner in which Darius managed to do so indicates a brilliant military strategist. He was able to mobilize an entire royal army in a six-month time period. Furthermore, even though his army was decimated at Issus, Darius did not give up hope. In fact, within two years he had rebuilt the Persian army. The significance of this ability is revealed when one realizes that even during the height of the Persian Empire, King Xerxes took twice as long to accomplish the same task. If Darius was able to accomplish that task within two years without the support of the Greek city-states, it stands to reason that he could have done so in a much shorter time with their…