The Battle of the Granicus was the first fight during Alexander‘s campaign against the Achaemenid Empire, which had, since the death of Artaxerxes III Ochus in 338, been torn apart by civil war. Artaxerxes IV Arses – his throne name is recorded in the famous Xanthus trilingue – had been forced to deal with insurrections in Babylonia (Nidin-Bel), Egypt (Chababash), and Armenia (Artašata). In the end, Artaxerxes IV was replaced by the rebel satrap of Armenia, who was to rule as Darius III.
During this civil war, the western territories had been attacked by a Macedonian army commanded by Parmenion and Attalus. The Greek mercenary leader Memnon of Rhodes and the western satraps had been able to repell the invaders, but in the spring of 334, Alexander arrived with the main force. What happened at the Granicus, is something you can read here.