Hesse-Kassel was a landgraviate in Germany that existed from 1567 to 1803, being a state within the Holy Roman Empire after the dissolution of the Landgraviate of Hesse in 1567. Hesse-Kassel was one of the Protestant states that sided with the Protestant Union during the Thirty Years' War, and it formed an alliance with Great Britain during the Seven Years' War of 1756-1763 against the Kingdom of France. After the Seven Years' War, King George III of Britain was impressed with the military capabilities of the German states, and King George decided to purchase German mercenaries from princes such as the Landgrave of Hesse-Kassel; the German mercenaries were nicknamed "Hessians", as many of them came from Hesse-Kassel. Hesse-Kassel was conquered by Westphalia in 1803 during the Napoleonic Wars, but was restored at the Congress of Vienna in 1814 as the Electorate of Hesse.