The key to the success of Napoleon’s Grand Armée was his organizational innovation of making corps under his command self-sufficient armies unto themselves. On average, they numbered 20,000 to 30,000 men, usually commanded by a marshal or senior general, and were capable of fighting independently. Each was comprised of two or more infantry divisions of about 12,000 men, a brigade of cavalry (about 2,500 men) and six to eight companies of artillery (each about 100 to 120 men). In addition, each corps had a company of engineers, plus a headquarters staff, medical and service units, and supply train for baggage and ammunition. Napoleon commanded that each army corps never be more than a day’s march — about 20 miles — away from each other so they could support each other on the battlefield. Each corps commander was relied upon to orchestrate the bodies of men under his command to facilitate Napoleon’s ever-changing tactics. Infantry, cavalry and artillery each had to play their part in order to win battles. Napoleon's Imperial Guard, some of the most feared soldiers in Europe, were used as the ultimate weapon to ensure victory.