Alexander ' s Balkan campaign The Balkan campaign of Alexander the Great took place in 335 BC , against a number of rebellious vassals of the Macedonian kingdom . Alexander successfully pacified each in turn , leaving him free to begin the long planned invasion of Persia . Background . 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 . When he found the Thessalian army occupying the pass between Mount Olympus and Mount Ossa , he had the men ride through Mount Ossa and , when the Thessalians awoke , they found Alexander at their rear . The Thessalians surrendered and added 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 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 , to either open the ranks or lay flat on the ground with their shields over them . The Macedonian archers opened fire and when the Macedonian infantry reached the top of the mountain they routed the Thracians . Meanwhile , a large Triballian army led by their king , Syrmus , advanced upon the Macedonian rear . 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 . The Macedonians marched to the Danube River 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 . Illyria . The Siege of Pelium was undertaken by Alexander the Great against the Illyrian tribes of what is modern day Albania . It was critical for Alexander to take this pass as it provided easy access to Illyria and Macedonia , which was urgently needed in order to quell the unrest in Greece at this time in Athens and Thebes . This was an important point of demarcation in Alexander ' s early reign , as it established him among the Danubian tribes to the north as a serious monarch to be reckoned with , just as he would later establish this precedent for the Greek city states under his hegemony . Taking this place allowed Alexander to march his army to southern Greece quickly , which would eventually result in the total destruction of Thebes . Thebes . The Battle of Thebes was a battle that took place between Alexander III of Macedon and the Greek city state of Thebes in 335 BC immediately outside of and in the city proper . After having been made Hegemon of the League of Corinth , Alexander had marched to the north to deal with revolts in Illyria and Thrace , which forced him to draw heavily from the troops in Macedonia that was keeping pressure on the Greek city states of the south that was keeping them in subjection . Although Alexander did not desire to destroy the city of Thebes , after sending several embassies requesting their submission on merciful terms , he was eventually forced to destroy the city by force . Aftermath . With the destruction of Thebes , the Macedonian possessions and vassals in Europe were once again quiscent to Alexander ' s rule . Alexander was now finally free to undertake the Persian campaign which had been planned for so long by his father .