Battle of Waterloo was fought b/w Napolean and Britain with its allies.Eton was a renowned Institute of that time.At that time discipline, fighting,Cricket and other physical activities were taught to the students to make them more masculine because the main motive of Eton institute was to develop students to serve as army for the nation.As England along with the allies were to fight with the Napolean they need as strong an army can so that they could defeat him.This is the reason why it is said that battle of Waterloo was won on the playing field of Eton.