William V allied himself with Gustavus Adolphus of Sweden and then France, losing most of Hesse-Kassel when Imperial troops invaded.

He died in exile in 1637, leaving his widow Amalie Elisabeth of Hanau-Münzenberg to act as regent for their eight-year-old son William VI.

On Frederick I's death in 1751, he was succeeded by his brother William VIII, who fought as an ally of Kingdom of Great Britain during the Seven Years' War.

When the American Revolutionary War broke out, Frederick II leased Hessian troops to Great Britain for service in America.

At the Congress of Vienna, a number of Napoleonic electorates were elevated to kingdoms, and William tried to secure recognition as King of the Chatti.

The rulers of the Electorate of Hesse became the only Prince-Electors in the German Confederation, even though there was no longer a Holy Roman Emperor for them to elect.

[5] Hesse-Kassel took the practice to an extreme, maintaining 5.3% to 7.3% of its population under arms in peacetime in its history, however was known to double that in times of war.

[7] During the American Revolutionary War, the Kingdom of Great Britain rented thousands of German troops to fight in the Thirteen Colonies, half of whom came from Hesse-Kassel and nearby Hesse-Hanau.

Due to this, American colonists in the colonies referred to all German troops serving with the British with the synecdoche "Hessian".

[8] The village of Hessen Cassel, Indiana, near Fort Wayne, founded by German immigrants, is named for the Landgraviate of Hesse-Kassel.