Battle of the Granicus
Background: After winning the Battle of Chaeronea in 338 BC, King Philip II of Macedon forced most of the Greek states into a military alliance, the Hellenic League. Its goal was to make war on the Persian Achaemenid Empire to avenge the second Persian invasion of Greece in 480 BC. He managed to convince the other Greek states to elect him as the leader of the League and started preparing for the war.
At the same time the Achaemenid Empire had been in crisis since the murder of its king Artaxerxes III in 338 BC. An important consequence of this was that Egypt again seceded from the empire. At this time there was no large Persian army in Asia Minor and the Persian fleet was not expected in the Aegean Sea soon. Philip most likely wanted to exploit the chaos in the empire, but could not launch a large-scale invasion of Asia Minor because his entire army was not yet ready. For this reason he probably sent a smaller part of his army which could be deployed. In the spring of 336 BC he ordered a Macedonian expeditionary corps of several thousand soldiers to land on the western coast of Asia Minor. The likely mission of this advance-guard was to conquer as much territory as possible, or at the very least establish a bridgehead on the Asian side of the Hellespont to facilitate the crossing of the main army later.
The advance guard was most likely led by Parmenion, Philip's best general, with Attalus as second in command. Initially the campaign was a success, with many of the Greek cities of western Asia Minor surrendering peacefully to the Macedonians. At the end of 336 BC this all changed. Philip was murdered, most likely in October 336 BC. Philip was succeeded by his son, Alexander III, who had to put down several revolts in the Balkans and Greece first to reassert Macedonian authority. This distracted him from the operations in Asia Minor. Meanwhile, Darius III had become the new Great King of the Achaemenid Empire, around the autumn of 336 BC. He managed to stabilize the empire and started a counter-offensive against the Macedonian expeditionary force.
Darius prioritized the suppression of the Egyptian revolt, which probably took place from the end of 336 BC to February 335 BC. When this was done he sent Memnon of Rhodes to Asia Minor at the head of a force of Greek mercenaries, who were infantry fighting as hoplites. Memnon defeated a Macedonian force at either Magnesia ad Sipylum or Magnesia on the Maeander, probably in early spring 335 BC. Later that year another Macedonian force was defeated by a Persian army in the Troad, possibly under the command of one or more Persian satraps. By the end of 335 BC most of the Greek cities had been restored to Persian control and the expeditionary force retained only Abydus and perhaps Rhoeteum.

Prelude: Once Alexander had defeated the rebellions in the Balkans and Greece, he marched his army to the Hellespont in early spring 334 BC. His army numbered c. 32,000 infantry and c. 5,000 cavalry. He arrived at Sestus twenty days later, where he split his army for the crossing of the Hellespont. The main part of the army was transported from Sestus to Abydus in Asia Minor, while Alexander crossed with the remainder of the infantry from Elaeus, landing near Cape Sigeum. After visiting Ilium, he passed Arisba, Percote, Lampsacus, Colonae and Hermotus.
Darius would have been informed about Alexander's movements for some time, maybe as early as the invasion force had left Macedon. This did not alarm the Great King yet, who left the defense of Asia Minor to his satraps there. The reasons for this may have been that Alexander had not proven himself as a commander abroad yet and that the Macedonian expeditionary force had been pushed back without much difficulty the previous year.
The Persian satraps and commanders were encamped near Zeleia with their army when they were informed of Alexander's crossing. The army was led by Arsites, the satrap of Hellespontine Phrygia, Spithridates, satrap of Lydia and Ionia, Arsames, satrap of Cilicia, Rheomithres, Petenes, Niphates and Memnon of Rhodes. While the ancient historians don't explicitly identify a commander-in-chief for the Persians, modern historians consider Arsites to have been in overall command because the enemy had invaded his territory.
A council was held by the Persians at Zeleia to discuss the state of affairs. The ancient historians Arrian and Diodorus Siculus claim that Memnon advised the Persians to avoid open battle with the Macedonians. He reasoned that the Macedonian infantry was superior to theirs and that Alexander was present in person, while Darius was not with them. He recommended a scorched earth strategy so that Alexander would not be able to continue his campaign due to lack of supplies. Arsites and the other Persians rejected Memnon's advice because they wanted to protect the property of their subjects. They also suspected Memnon of trying to prolong the war so that he could gain favor with Darius.
W. J. McCoy compares Memnon's advice with the wise counsel given by Greeks to Persians in the work of Herodotus. In Herodotus, the rejection of such counsel led to misfortune for the Persians and was employed to ridicule them. For this reason he suggests that Memnon's advice was an invention by Arrian and Diodorus. Pierre Briant argues that even if the debate took place as described, the relevance of Memnon's counsel was overstated because discussing strategy was not the purpose of the meeting. Most likely Darius had already ordered his satraps to engage Alexander's army in battle, so the council at Zeleia was held to discuss the tactics of the upcoming battle rather than the strategy of the war. After the council, the Persian army took up position at the eastern bank of the Granicus River and waited for Alexander's attack.
According to Arrian, Alexander heard of the location of the Persians from his scouts while he was moving towards the Granicus. He had his army arranged in battle formation. Parmenion was worried that the army could not cross the river with its front extended, and would have to cross over in column with its battle formation disrupted. Because this would make the army vulnerable to Persian attack, he advised Alexander that the army should encamp on the western bank and make an uncontested crossing tomorrow at dawn. Alexander did not want to wait and ordered an immediate attack.
These discussions on strategy and tactics between Parmenion and Alexander are a recurring motif in the writings of ancient historians. In these exchanges Parmenion is depicted as careful, while Alexander is willing to take more risks. Alexander always rejects the advice of Parmenion and is then successful in realizing his own bold plan, which the old general advised him against. This motif may have originated in the work of Callisthenes for the purpose of enhancing the heroism of Alexander. As a consequence, historians have suggested that the discussion may have been fictional.
The battle was fought in the Macedonian month of Daisios, May in the Gregorian calendar. Before the battle, some of the Macedonian officers voiced their reluctance to fight because religious custom required them to avoid battles during this month. Alexander solved this by declaring that the current month should be considered a second Artemision. This month preceded Daisios and did not carry a taboo on warfare.

Location: The Granicus is today called the Biga River (Turkish: Biga Çayı). Traditionally the ford where the battle took place has been located above the confluence of the river which runs through Gümüşçay and the Biga River, or further downstream on the Biga River. More recently N. G. L. Hammond proposed that at the time of the battle the Biga River possibly flowed further east in its valley. In modern times, evidence of an old riverbed can still be seen there. Furthermore, the ancient historians describe a long continuous ridge where the Greek mercenary infantry of the Persians was posted. Because there is only one candidate for this ridge, Hammond has placed the ford between the modern villages Gümüşçay (formerly called Dimetoka) and Çeşmealtı. This theory found support in later scholarship on the battle. The modern observations that the river is not difficult to cross contradicts the ancient historians, who emphasized the hardship of the fording of the river. The ancient historians might have exaggerated to make Alexander's accomplishment seem more formidable.

Opposing forces: 

Macedonian army: The Macedonian army consisted mostly of infantry. The heavy infantry numbered 12,000 and included the Foot Companions and a smaller group of elite hypaspists. The light infantry totalled 1,000 and contained archers and elite Agrianian javelin-men. The heavy cavalry was made up of 1,800 elite Companion cavalry, 1,800 Thessalian cavalry and 600 Greek allied cavalry. The light cavalry numbered 900 in total and was composed of prodromoi, Paeonians and Thracians. Alexander also possessed Greek allied and mercenary infantry, but he had marched to the Granicus without them.