After the assassination of Alexander's father, Greek nations rebelled. Among the most prominent was Thebes. Alexander performed an astonishing quick march and defeated the city-state.
This was Alexander's 1st important victory against the Persians. He surprised Persian forces camped on other side of river Granicus by attacking late in the day when most commanders would have been reluctant for a major military movement.
This was Alexander's first encounter with Persian King Darius. Though his army had less soldiers than the enemy, Alexander forced the Persians into a disadvantageous position and achieved victory.
This was a major victory for Alexander against Darius. Even when his army was outnumbered 3 to 1, Alexander tactically created gap in Persian ranks and Darius and his retinue had to retreat. Watching the king running away, the Persian army crumbled.
Alexander turned a near defeat into victory in this battle by pinning the Persian army between his main force and a smaller detachment bravely led personally by him.