Leaving Egypt in 331 BC, Alexander marched eastward into Mesopotamia (now northern Iraq) and again defeated Darius, at the Battle of Gaugamela. Darius once more fled the field, and Alexander chased him as far as Arbela. Gaugamela would be the final and decisive encounter between the two. Darius fled over the mountains to Ecbatana (modern Hamedan), while Alexander captured Babylon.
Alexander earned the epithet "the Great" due to his unparalleled success as a military commander. He never lost a battle, despite typically being outnumbered. This was due to use of terrain, phalanx and cavalry tactics, bold strategy, and the fierce loyalty of his troops. The Macedonian phalanx, armed with the sarissa, a spear 6 metres (20 ft) long, had been developed and perfected by Philip II through rigorous training, and Alexander used its speed and maneuverability to great effect against larger but more disparate Persian forces. Alexander also recognized the potential for disunity among his diverse army, which employed various languages and weapons. He overcame this by being personally involved in battle, in the manner of a Macedonian king.
In his first battle in Asia, at Granicus, Alexander used only a small part of his forces, perhaps 13,000 infantry with 5,000 cavalry, against a much larger Persian force of 40,000. Alexander placed the phalanx at the center and cavalry and archers on the wings, so that his line matched the length of the Persian cavalry line, about 3 km (1.86 mi). By contrast, the Persian infantry was stationed behind its cavalry. This ensured that Alexander would not be outflanked, while his phalanx, armed with long pikes, had a considerable advantage over the Persian's scimitars and javelins. Macedonian losses were negligible compared to those of the Persians.
At Issus in 333 BC, his first confrontation with Darius, he used the same deployment, and again the central phalanx pushed through. Alexander personally led the charge in the center, routing the opposing army. At the decisive encounter with Darius at Gaugamela, Darius equipped his chariots with scythes on the wheels to break up the phalanx and equipped his cavalry with pikes. Alexander arranged a double phalanx, with the center advancing at an angle, parting when the chariots bore down and then reforming. The advance was successful and broke Darius' center, causing the latter to flee once again.
When faced with opponents who used unfamiliar fighting techniques, such as in Central Asia and India, Alexander adapted his forces to his opponents' style. Thus, in Bactria and Sogdiana, Alexander successfully used his javelin throwers and archers to prevent outflanking movements, while massing his cavalry at the center. In India, confronted by Porus' elephant corps, the Macedonians opened their ranks to envelop the elephants and used their sarissas to strike upwards and dislodge the elephants' handlers.