The Royal Prussian Army (1701–1919, Königlich Preußische Armee) served as the army of the Kingdom of Prussia. It became vital to the development of Prussia as a European power.
The Prussian Army had its roots in the core mercenary forces of Brandenburg-Prussia 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.
The Prussian army had become outdated and under-resourced by the beginning of the Napoleonic Wars, and France defeated Prussia in the War of the Fourth Coalition in 1806. However, under the leadership of Gerhard von Scharnhorst, Prussian reformers began modernizing the Prussian Army, which contributed greatly to the defeat of Napoleon during the War of the Sixth Coalition. Conservatives halted some of the reforms, however, and the Prussian Army subsequently became a bulwark of the conservative Prussian government.
In the 19th century, the Prussian Army fought successful wars against Denmark, Austria, and France, allowing Prussia to unify Germany, establishing the German Empire in 1871. The Prussian Army formed the core of the Imperial German Army, which was replaced by the Reichswehr after World War I.
The army of Prussia grew out of the united armed forces created during the reign of Elector Frederick William of Brandenburg (1640–1688). Hohenzollern Brandenburg-Prussia had primarily relied upon Landsknecht mercenaries during the Thirty Years' War, in which Brandenburg was devastated. Swedish and Imperial forces occupied the country. In the spring of 1644, Frederick William started building a standing army through conscription to better defend his state.
By 1643–44, the developing army numbered only 5,500 troops, including 500 musketeers in Frederick William's bodyguard.