A division is a large military unit or formation, usually consisting of between 10,000 and 20,000 soldiers. Infantry divisions during the World Wars ranged between 8,000 and 30,000 in nominal strength.
In most armies, a division is composed of several regiments or brigades; in turn, several divisions typically make up a corps. Historically, the division has been the default combined arms unit capable of independent operations. Smaller combined arms units, such as the American Regimental combat team (RCT) during World War II, were used when conditions favored them. In recent times, Modern Western militaries have begun adopting the smaller Brigade combat team (similar to the RCT) as the default combined arms unit, with the Division they belong to being less important.
While the focus of this article is on army divisions, in naval usage division has a completely different meaning, referring to either an administrative/functional sub-unit of a department (e.g., fire control division of the weapons department) aboard naval and coast guard ships, shore commands, and in naval aviation units (including navy, marine corps, and coast guard aviation), to a sub-unit of several ships within a flotilla or squadron, or to two or three sections of aircraft operating under a designated division leader.
In administrative/functional sub-unit usage unit size varies widely, though typically divisions number far fewer than 100 people and are roughly equivalent in function and organizational hierarchy/command relationship to a platoon or flight.
In the West, the first general to think of organising an army into smaller combined-arms units was Maurice de Saxe (d. 1750), Marshal General of France, in his book Mes Rêveries. He died at the age of 54, without having implemented his idea. Victor-François de Broglie put the ideas into practice. He conducted successful practical experiments of the divisional system in the Seven Years' War.
The first war in which the divisional system was used systematically was the French Revolutionary War. Lazare Carnot of the Committee of Public Safety, who was in charge of military affairs, came to the same conclusion about it as the previous royal government, and the army was organised into divisions.
It made the armies more flexible and easy to maneuver, and it also made the large army of the revolution manageable. Under Napoleon, the divisions were grouped together into corps, because of their increasing size. Napoleon's military success spread the divisional and corps system all over Europe; by the end of the Napoleonic Wars, all armies in Europe had adopted it.