Darius III (c. 380 - 330 BC) was king of the Achaemenid Empire from 336 to 330 BC. He was a great-grandson of Artaxerxes II but was not expected to succeed to the throne. He had a distinguished career in the military. However the machinations of Bagoas, Vizier of the Empire, resulted in Darius' great-uncle Artaxerxes III and all his male heirs being assassinated within a period of two years. Bagoas chose to elevate Darius to the throne, in the apparent belief that a king with no administrative experience would be easier to control. Instead Darius managed to become the de facto ruler and had Bagoas killed.
Darius III had to face the ambitions of various satraps (regional governors) for autonomous rule and internal revolts, particularly in the unstable province of Egypt. Also, Philip II of Macedon was organizing an alliance of Greek states against the Achaemenid Empire. In 336 BC, an advance force under generals Attalus and Parmenion had already captured areas of Anatolia and waited for Philip to lead reinforcements. The assassination of Philip II by Pausanias of Orestis, a member of the royal bodyguard, somewhat delayed the planned invasion. Alexander the Great, Philip's son and heir, had to struggle against rival contenders for the throne of Macedon and to restore Macedonian domination of the Greek cities.
By 334 BC, Alexander was the indisputable king of Macedon and head of the League of Corinth. The League was a Greek alliance including almost all states of the Southern Balkans and the Aegean Sea. Major exceptions were Sparta and the cities of Crete. Alexander personally led the invasion of Achaemenid territories. The Battle of Granicus river (modern Biga Çayı, Turkey) established Alexander's control over Asia Minor (Anatolia).
In 333 BC, Darius decided to personally lead his troops in the Battle of Issus (currently in the borderline between Turkey and Syria). It was a great defeat for Darius who lost an estimated 1/5 of his forces and had to retreat further east. In his escape, Darius allowed Alexander to capture his nearby headquarters. Not only did Alexander capture personal items of his rival, the headquarters housed Darius' most of the royal family. Alexander had Darius' mother, wives and children under his control. They were treated well but would never be returned to Darius.
The two rivals would not meet in battle again until 331 BC. Alexander had spent two years taking over the western provinces of the Achaemenid Empire while Darius was reorganizing his army in the eastern ones. The two kings led their armies in the Battle of Arbela/Gaugamela (named thus because the location of Gaugamela was nearby the city of Arbela). The battle was fought near the still extant city of Arbil and also close to the location of modern Mosul. Both cities are currently located in northern Iraq. Darius seems to have lost the majority of his army in the battle and escaped with the remnants. Alexander was free to head to Babylon, one of the major cities and major treasuries of the Empire.
Darius tried to organize his army but there were few available recruits. Meanwhile Alexander had proceeded from Babylon to Susa and Persepolis. Thus Darius had lost his capital and effectively the entire province of Persia, the birthplace of his dynasty. Alexander attempted to incorporate the surviving administrative and military elite into his own forces and was successful enough to undermine Darius' recruiting efforts. At this point Darius was betrayed by his subordinate Bessus, satrap of Bactria (most of the satrapy currently held by Afghanistan). Bessus took the king prisoner and led the remnants of the army to Bactria, placing them under his personal control.
Alexander was still determined to capture Darius alive and invaded Bactria. Darius found himself a pawn contested between Bessus and Alexander, both of the men having claimed the Achaemenid throne for themselves. Bessus was defeated and had to retreat. He chose to assassinate Darius before leaving. Alexander only captured the corpse of the King. He gave Darius a magnificent funeral. In 324 BC, Alexander married Statira, one of Darius' daughters, to further establish his claim of being the legal heir to the Achaemenids.