Before Napoleon, armies moved as a single, sluggish block. Napoleon broke his army into "mini-armies" (Corps) of 20,000 to 30,000 men, each complete with its own infantry, cavalry, artillery, and staff.
This allowed his army to move along multiple roads simultaneously, foraging for food and moving twice as fast as his enemies who were stuck on a single route.
When one Corps encountered the enemy, it could hold them off for a day while the other Corps converged from all directions, surrounding the confused opponent.