The Royal Prussian Army (German: Königlich Preußische Armee) served as the army of the Kingdom of Prussia. It became vital to the development of Brandenburg-Prussia as a European power. The Prussian Army had its roots in the core mercenary forces of Brandenburg during the Thirty Years' War of 1618-1648. Elector Frederick William developed it into a viable standing army, while King Frederick William I of Prussia dramatically increased its size and improved its doctrines. King Frederick the Great, a formidable battle commander, led the disciplined Prussian troops to victory during the 18th-century Silesian Wars and greatly increased the prestige of the Kingdom of Prussia.