How a twenty-year-old king launched the most ambitious military campaign in ancient history
In short: When Alexander the Great inherited the Macedonian throne in 336 BCE, he faced an empire in flux and a massive debt. By 334 BCE, he had mobilized a sophisticated, multi-ethnic army to invade Persia. His victory at the River Granicus signaled the beginning of a campaign that reshaped the ancient world.
Alexander’s path to the Persian invasion was forged in the shadow of his father, Philip II. After Philip’s assassination in 336 BCE, Alexander secured his position by eliminating rivals and consolidating power across Greece. Appointed generalissimo by the League of Corinth, he inherited not just a kingdom, but a formidable military machine. Driven by the need to fund his army and settle a 500-talent debt, Alexander turned his gaze toward the perceived vulnerability of the Achaemenid Empire, a weakness previously exposed by Greek mercenaries and Spartan campaigns.
In the spring of 334 BCE, Alexander crossed the Dardanelles with a force of roughly 35,000 men, including 30,000 infantry and over 5,000 cavalry. His army was a marvel of balanced warfare, integrating light-armed archers and javelin warriors with the heavy infantry phalanx and elite hypaspists. Unlike typical armies of the era, his expeditionary force included engineers, scientists, and historians, suggesting he envisioned an operation of unprecedented scale and duration.
His first major test came at the Granicus River, where he confronted Persian satraps near the Sea of Marmara. The Persians attempted to trap him in the river crossing, but the Macedonian line held, leading to a decisive victory. This success opened western Asia Minor to his forces, where he dismantled existing tyrannies and installed democracies—a sharp departure from his earlier, brutal suppression of revolts in Greece. By the time he moved forward, Alexander had already begun to transform from a Macedonian king into the architect of a new, Hellenistic world.
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Sources
- Summary by Intellect · source: The Greatest General in History? Alexander invades the Persian Empire
- Also: Alexander the Great | Empire, Death, Map, & Facts | Britannica
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