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Alexander's impulses get the better of him, and he pursues the fleeing Darius instead of returning to rescue Parmenion.
This results in Parmenion's flank being overwhelmed and most of Alexander's army killed or captured.
Alexander meanwhile captures Darius, and the Persian King dies, either fighting, committing suicide, or executed - doesn't really matter.
What now though? Alexander only has his companion cavalry, though the Persians are leaderless...