In 336 BC, Philip attended a public ceremony at Agai. As he entered a theater, unarmed, one of his own bodyguards approached him and struck. Philip was stabbed and killed on the spot. The attacker attempted to flee, but was killed immediately. Philip was dead. Alexander, at 20 years old, was declared king by the army and the Macedonian nobles. Before launching a campaign into Asia, he needed to secure his borders. Tribes in Thrace and Illyria were preparing to revolt, and leaving them unchecked risked an uprising behind him once he moved deep into Persian territory. In 335 BC, he led an army out of Amphipolis and advanced toward the Hemos Mountains. As his forces approached a narrow mountain pass, Thracian tribes had already taken position along the high ground. They prepared an ambush using wagons lined along the slope above the road. Alexander spotted it and ordered his men to part ways, allowing the speeding wagons to pass by. Once the wagons had passed, Alexander ordered an immediate counterattack. Approximately 1,500 were killed during the retreat. Few prisoners were taken, but the Macedonians captured their camp followers. including women and children, along with their supplies. After breaking through the mountains, Alexander reached the River Ister, a natural barrier guarded by a large Gete force waiting on the far bank. Under cover of darkness, part of his army slipped across the river undetected. At dawn, they advanced in formation, and the Gete did not hold. They broke almost immediately, abandoning their city as Alexander drove forward. The settlement was taken and destroyed, not to hold, but to send a message. No distance, no river, no barrier would stop him, and the message spread. Following the destruction of the Getae city, nearby tribes began sending envoys to negotiate rather than resist. Among them were representatives of Celtic groups from the west near the Ionian Gulf. They had not been defeated and did not approach as subjects, but as independent tribes observing the expansion of Macedonian power. The Celts were physically large and carried themselves with confidence. Their societies were tribal, organized around warrior elites, with an emphasis on personal valor and reputation in battle. Their style of warfare placed less importance on rigid formation and more on individual courage and direct confrontation, in contrast to the structured systems of the Greek world. accepted their terms and later referred to them as boastful, but no action was taken against them. With the northern tribes subdued and no immediate threats along the Danube, Alexander the Great turned west. Reports reached him that a new uprising had formed, he advanced toward Pelium and found