During the Thirty Years' War, Calvinist Hesse-Kassel proved Sweden's most loyal German ally. Landgrave Wilhelm V, and after his death in 1637, his widow, continued to support the French and Swedes throughout the war, and throughout maintained its own army, garrisoning many strongpoints, even while Hesse-Kassel itself was occupied by Imperial troops.
During the later 17th and 18th centuries, the Landgraves of Hesse-Kassel became famous for hiring out their army as mercenaries. Landgrave Frederick II, notably, hired out his troops to his nephew George III of Great Britain to suppress the rebellion of Britain's American colonies.
The Reichsdeputationshauptschluss (Principal Decree of the Imperial Deputation) of 1803 granted Landgrave Wilhelm IX the position of an Imperial Elector (Kurfürst) and took the title Wilhelm I, Elector of Hesse, although the state was still usually referred to as Hesse-Kassel. In 1806, Wilhelm I was dispossessed by Napoleon for his support of Prussia, and Kassel became the capital of a new Kingdom of Westphalia under Napoleon's brother Jerome. The Elector was restored as a result of Napoleon's defeat in 1813, and although the Holy Roman Empire was now defunct, Wilhelm clung to his title of Elector, hoping it would give him pre-eminence over his cousin, the Grand Duke of Hesse-Darmstadt.
Wilhelm's grandson, Elector Friedrich Wilhelm, sided with Austria in the Austro-Prussian War, and as a result, his lands were annexed by Prussia after the war, forming, along with Nassau and Frankfurt-am-Main, the new Prussian province of Hesse-Nassau.
In 1918, Prince Friedrich Karl of Hesse-Kassel, younger brother of the head of the house and a brother-in-law of Kaiser Wilhelm II, was selected by the Finnish pro-German government to be King of Finland, but he never reigned. In 1968, the head of the House of Hesse-Kassel became the Head of the entire House of Hesse due to the extinction of the Hesse-Darmstadt line.
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2 Electors of Hesse(-Kassel), 1803-1866
3 Heads of the House of Hesse-Kassel, 1866-present
Landgraves of Hesse-Kassel, 1568-1803
Electors of Hesse(-Kassel), 1803-1866
to the Kingdom of Westphalia, 1806-1813
Heads of the House of Hesse-Kassel, 1866-present