A German electoral state centered on Kassel, elevated from the Landgraviate of Hesse-Kassel in 1803, briefly absorbed into the Napoleonic Kingdom of Westphalia (1807-1813), restored in 1814, and annexed by Prussia in 1866 following the Austro-Prussian War.
Annexed by Prussia in 1866 after siding with Austria in the Austro-Prussian War (Seven Weeks' War).
The original cited author's own scholarly study of the Hesse-Kassel ruling house across the Landgraviate-to-Electorate transition, back in print via Princeton Legacy Library.