In the field of Greek military history and tactics, there is one formation that astounds with its simplicity and also with its effectiveness: the Oblique-Heavy Left.
In traditional phalanx warfare (a great source-book for which is The Western Way of War by Victor Davis Hanson), the king or general and his bodyguard anchored the right wing. Both forces lined up parallel to each other, the strongest troops on the right, weakest on the left. This was because the hoplon (shield) protected the left half of the body and the right half of the person next in line. The person on the far right was only half covered, while the left was protected. Thus the tendency was to duck to the right to stay under the shield of the neighbor and would result in the line of battle drifting in that direction. The left flank would then be turned as the right of the enemy also drifted and so on. By placing the best troops on the right, it could counteract the drift as it anchored the line.
Since the enemy commander held that side, Epaminondas took an earlier innovation of a deep phalanx (traditional was 8 ranks, Pagondas used 25 at Delium) and further weighted his left flank, spearheaded by his best troops. 50 ranks deep at Leuctra, his left flank started closer than his right flank, while his lighter right was instructed to close with the enemy slower, allowing the left to hammer the enemy’s best troops first. Since hoplite warfare devolved into a shoving match, having the weight of 50 men, instead of the weight of 8 made a difference. Once the commander went down, the army ceased to fight.
Epaminondas saw a means to apply force more efficiently and in doing so changed the power structure of Greece. Simple as it was, the Oblique-Heavy left (and the surety that the Spartans were not invincible) was a revolutionary development.
Of course this did not stop the innovator from dying in battle at Mantinea nine years later, which in part allowed a power vacuum to develop, into which Philip II’s Macedonia stepped.