The Battle of Leuctra was fought in 371 BC, during the Greek civil war between Sparta and its allies vs. Thebes and the Boeotian Confederacy. Sparta came onto the field with 2,000 hoplites under the command of King Cleombrotus on the right flank, and 10,000 men from their various Peloponnesian allies on the left flank. The Thebeans, under Epaminondas’ command, brought 3,000 men. 300 hundred of which are the Sacred Band, an elite unit of Thebean hoplites, Together, these 3,300 troops formed the line’s left flank. Their right flank was composed of 3,000 Boeotian allies.
The Thebeans weighted their left flank to about fifty shields deep, instead of the traditional eight-man deep phalanx, and angled their right flank so that the weighted phalanx would engage first. As the armies advanced towards each, other the Spartans tried to out flank the deep Thebean phalanx on their right. This failed as the Thebeans smashed into their line as they were trying to maneuver around. Cleombrotus was killed in this battle and the Spartan line was broken.Ancient sources disagree as to why the Thebeans were victorious in this battle. Nevertheless, modern historians agree that it was Epaminondas’ weighted phalanx that had the ability to negate the Spartans’ maneuvers and slay their king. Thus, this innovation secured Thebean victory at the Battle of Leuctra (Hanson, Battle of Leuctra).
This battle marked a dramatic end to the Spartan military dominance of the past three centuries. This defeat of Sparta allowed Thebes to invade and effectively end Sparta’s position as one of the great Greek powers (Hanson, Battle of Leuctra).