Balkan campaign of Alexander the Great

In 336 BC, while attending the wedding of his daughter by Olympias, Cleopatra and Olympias' brother, Alexander I of Epirus at Aegae, Philip II was assassinated by the captain of his bodyguard, Pausanias. Philip's son, and previously designated heir, Alexander was proclaimed king by the Macedonian noblemen and army.

News of Philip's death roused many states into revolt including Thebes, Athens, Thessaly and the Thracian tribes to the north of Macedon. When news of the revolt reached Alexander he responded quickly. Though his advisors advised him to use diplomacy, Alexander mustered the Macedonian cavalry of 3,000 men and rode south towards Thessaly, Macedon's immediate neighbor to the south. Finding the Thessalian army occupying the pass between Mount Olympus and Mount Ossa, he had the men ride through Mount Ossa and position themselves at the rear of the enemy encampment. Upon discovering this maneuver, the Thessalians surrendered, adding their cavalry to Alexander's force as he rode down towards the Peloponnese.

Alexander stopped at Thermopylae, where he was recognised as the leader of the Sacred League before heading south to Corinth. Athens sued for peace and Alexander received the envoy and pardoned anyone involved with the uprising. At Corinth, he was given the title 'Hegemon' of the Greek forces against the Persians. While at Corinth, he heard the news of the Thracian rising to the north.

Thrace

Before crossing to Asia, Alexander wanted to safeguard his northern borders and, in the spring of 335 BC, he advanced into Thrace to deal with the revolt, which was led by the Illyrians and Triballi. He was reinforced along the way by the Agriani, a Thracian/Paeonian tribe under the command of Alexander's friend, Langarus. The Macedonian army marched up to Mount Haemus, where they met a Thracian garrison manning the heights. The Thracians had constructed a palisade of carts, which they intended to throw upon the approaching Macedonians. Alexander ordered his heavy infantry to march in loose formation and, when the carts were thrown, either to open the ranks or lie flat on the ground with their shields over them. When they came into range the Macedonian archers fired at the Thracians, disrupting their battle line, all the while the Macedonian heavy infantry marched on. When they reached the top of the ridge they dislodged and routed the Thracians.

Descending on the lands of the Triballians, Alexander found out that their king, Syrmus, had gathered his population near the River Lyginus. Hearing of the size of the Macedonian army Syrmus retreated to Peuce Island, an island in the middle of the Danube. When the Macedonian army marched on Syrmus, a second Triballian force advanced upon their rear. Alexander turned his army around and advanced on the second Triballian force. The Triballians retreated to a gorge, where they were drawn out by Alexander's light infantry. On the open ground, they were crushed by Alexander's infantry and cavalry, leaving behind 3,000 dead. Alexander brought ships up the Danube from Byzantium to use in an assault on the island. The attempts failed due to the swift current of the river, steep banks, and fierce defense. Eventually the Macedonians abandoned their attacks on Peuce and instead crossed into the territory of the Getae.

The Macedonians marched to the Danube where they encountered the Getae tribe on the opposite shore. As Alexander's ships failed to enter the river, Alexander's army made rafts out of their leather tents. A force of 4,000 infantry and 1,500 cavalry crossed the river, to the amazement of the Getae army of 14,000 men. The Getae army retreated after the first cavalry skirmish, leaving their town to the Macedonian army.