In early summer 331 B.C., Alexander took his entire army north-east through Syria, reaching the Thapsacus on the Euphrates no early than July 10. His objective was to take Babylon, and Darius knew it. He could tell because Babylon was the economic center of the Persian empire and it had a strategic bastion protecting Susa, Persepolis, and the eastern provinces.

Darius was pretty sure about how Alexander was planning to take Babylon, too. He thought that Alexander would go straight down the eastern bank of the Euphrates, just as Cyrus did. Darius hoped to defeat him at Cunaxa, a city near the bank of the Euphrates. He thought that Alexander would go this way because he knew Alexander stuck hard, fast, and with maximum economy.

Alexander's troops, he assumed, would reach Cunaxa hot and exhausted. Darius would order the general Mazaeus advancing force simply retreat before the invader, and burn all of the crops and fodder as he went. Between this scorched earth policy and the blazing Mesepotamian sun, the Macedonians would be easily defeated, just like what happened to Cyrus and Cunaxa.

But instead of marching downstream like Darius had expected, Alexander went in a north-east direction across the Mesepotamian plain. Mazaeus watched, horror struck. He then rode the 440 miles back to Babylon with the news. All hope of a second Cunaxa was shattered.

When Darius heard this news he decided to try to hold Alexander at the Tigris. This was a very bold and risky plan because no one could be sure where Alexander might turn up. The entire plan depended on perfect coordination between Mazaeus, his scouts, and the command headquarters. After he had made this plan, Darius got his forces to Arbela and prepared to go to Mosul.

While the Persians prepared for this new strategy, Alexander captured a few of Darius' men. Under interrogation they told the entire Persian plan and the size and composition of Persia's army. When he found out this information, Alexander turned into the direction of Abu Wajnam, 40 miles to the north.

The Macedonians reached Abu Wajnam on September 18 without opposition. A few scouts fled to tell Darius, who was across the Greater Zab and approaching Mosul. When Darius learned that the Tigris was no longer separated his army and Alexander's he once again changed his plans. He decided to have the battle at the village of Guagamela between the Khazir River and the ruins of Nineveh. It was a good place for cavalry and chariot maneuvers, Darius' strongest unit. The cavalry sector outflanked Alexander's left flank.

Darius brought his troops to Guagamela and sent sappers to clear the plain. He did not, however, occupy the low hills some three miles to the north-west. This was a big mistake on Darius' part because from these hills Alexander's scouts could observe everything the Persians did.

After he had crossed the Tigris, Alexander made contact with a regiment of Mazaeus' camp. Mounted soldiers under the command of Paeoniar were sent to deal with the Persian fleet. The Persian Ariston speared a Macedonian colonels head and presented it to Darius.

Four days after this, Ariston's cavalry was spotted again. Alexander made a quick cavalry raid on the Persians, getting a few prisoners. Alexander interrogated them and received the information he needed.

After this, he gave his troops another four days to rest because he wanted them to be fresh for the coming battle. There camp was guarded by sentries, ditches, and a palisade(a fence of pickets). While the troops were resting, Darius' agents tried to sneak in notes telling the troops that they would do well to kill Alexander. These notes were intercepted and destroyed.

Alexander spent most of the daytime in September 29 331 B.C. around Darius' lines with a huge cavalry escort examining Darius' forces. The Persian's let him do this unopposed. That night, while his men ate and slept, Alexander stayed in his tent drawing up tactic after tactic. He finally drew up a master plan and went to bed. The next morning, Alexander kept on sleeping. He slept through breakfast. Finally, after breakfast, someone woke him. When this man inquired why Alexander had slept so late, Alexander merely said that he had been tired.

Because of Guagamela's geographic conditions and the numeric difference between his army and Darius', Alexander made sure to protect his rear and flanks and make his cavalry look weaker than it really was. Alexander stationed a powerful force of mercenaries on the right flank and masked them with cavalry squadrons. He pushed his left flank 45° from his main battle line. His lead infantry and remaining Greek mercenaries were stationed to cover the rear.

To reduce the odds against him and make an opening for his charge, Alexander planned to get as much of Darius' cavalry away from the center and into his flank guards. When the flanks were committed he would strike the center. This was an excellent plan and would be used centuries later by another great conqueror, Napoleon.

The Persians' left outflanked Alexander's so much that the Persian cavalry was almost opposite the Persian headquarters post. Because of the Persians' numerical superiority, this would be a hard battle.

Neither side wanted to act first, but Darius finally did. Trying to halt the drift of his left to dangerous ground he ordered an attack on Alexander's right flank. The Macedonians advanced with their left flank back, trying to get the Persian right into a premature flank engagement.

Soon after the Persians had attacked, Alexander added some rangers to the battle. Just then, he saw a gap in Darius' center. Because of this, he gathered his forces in a wedge formation and charged. In two or three minutes, the whole course of the battle was changed.

While this was happening, Bessus, still completely engaged with Alexander's right, found his flank dangerously exposed by Alexander's charge. By then Bessus had completely lost contact with Darius and was afraid that at any moment Alexander's wedge could come to his right side and take his rear guard. Because of this fear, he sounded retreat and began to withdraw.

To stop this increasing pressure, Darius' cavalry commander brought up more men to roll up Alexander's right flank. He was probably still unaware of the 6,000 Macedonian mercenaries behind the cavalry. This was just the move Alexander was waiting for, and once the Persian cavalry was engaged Alexander fed in further units from his flank guard. Around this time, Alexander's cavalry, which, as was stated earlier, was about 1,100 strong, held nearly ten times its own strength.

While this was happening, Darius launched his chariots. They were highly ineffective. Alexander's screen of lightly armored troops in front of the main line slaughtered the horses with javelins and stabbed the drivers as they rode past. The well drilled ranks of Alexander's rear phalanx opened, and the survivors of the chariot slaughter were rounded up.

By the time this slaughter was done, almost all of the Persian army was engaged in the battle. Parmenio was fighting a defensive against Mazaeus on the left while on the right Alexander had just added more rangers to hold Bessus. Then Bessus and the rest of the army began to withdraw. Darius, as he often did, fled. He was barely able to before he was sucked further into the battle, and rushed toward Arbela, dust clouds swirling behind his chariot. Mazaeus, on the Persian right, saw him and broke off. Bessus was already withdrawing, and the entire Persian line was chopped to bits.

After his defeat in this battle, Darius made a few attempts to reorganize and rearm his troops. He also sent a few nervous notes to his governors and generals in Bactria urging them to remain loyal. After Guagamela, though, Darius lost his nerve and never recovered it.

While Parmenio rounded the Persian luggage up with its animals and supplies, Alexander rode on, hoping to overtake Darius' party. He rested an hour or two, and resumed the chase at midnight. When he reached Arbela at dawn he found Darius gone after they had gone some 75 miles during the night's chase.

Alexander managed to figure out how Darius had managed to escape from Guagamela. He and his followers fled headlong into Arbela, not even bothering to break river bridges as they went. At Arbela they were joined by Bessus, a few survivors from the Royal Guard, the Bactrian cavalry, and 2,000 Greek mercenaries. Soon after midnight he set out, taking the eastern road through the Armenian mountains. They eventually hit Ecbatana from the north. They stopped here for a while.

After fleeing from the battle, Darius left behind his chariot, bow, and about 4,000 talents in coined money. This was a substantial amount of money, equal to about 7.5 million dollars today. Alexander pursued Darius, now a fugitive in his own country, for hundreds of miles. When he finally caught up to him, he found the Persian king dead in his coach, assassinated by his own officers. Alexander had the assassins executed and gave Darius a royal funeral.

After Alexander's victory, the entire Persian empire was split in two. The ruler's authority was ripped to shreds. The people of the empire were no longer united behind the Persian cause. Because of this, Alexander could proclaim himself the king of the empire in place of Darius, and no one could stop him. From Arbela, Alexander went to Babylon, which was acquired because of his victory in the battle of Guagamela.