Hesse-Kassel, former landgraviate of Germany, formed in 1567 in the division of old Hesse.
In 1567 Hesse was partitioned among four sons of Landgrave Philip the Magnanimous, Hesse-Kassel going to William IV the Wise. Hesse-Kassel was the largest, most important, and most northerly of the four Hesse landgraviates. The landgrave William IV introduced sound financial management and a pacific foreign policy. Under his successors, Hesse-Kassel became Calvinist. It fought on the side of the Swedes in the Thirty Years’ War and on the side of Great Britain in the Seven Years’ War. During the 18th century Hessian troops fought in nearly every European and Turkish campaign, and in 1776–86 as many as 17,000 Hessian mercenaries fought for the British in the American Revolution.
In 1803 Hesse-Kassel was raised to the status of an electorate, and it was to remain the only territory so styled after the end of the Holy Roman Empire (1806). The elector William I (reigned 1785–1821) pursued a policy of neutrality toward Napoleon, who nevertheless occupied Hesse-Kassel after the Battle of Jena (1806) and in 1807 united it with the Kingdom of Westphalia. In 1815 Hesse-Kassel regained its independence, but the elector William I and his successor William II (reigned 1821–47) were reactionaries who overturned the liberal reforms made in Hesse-Kassel previously by the French. The electors continuously quarreled with liberal reformers in the Diet (legislative assembly), and in 1831 revolutionary action compelled William II to turn over control of the government to his son Frederick William as regent. Frederick William became elector in 1847 and agreed to the liberal reforms adopted by the revolutionary government in 1848, but in 1850 he sided with the Austrians and repudiated the reforms, adopting a reactionary policy.
In the Seven Weeks’ War between Prussia and Austria (1866), Frederick William sided with Austria, against the wishes of the majority of the Diet. The country was then annexed by Prussia and the elector exiled. In 1867 Hesse-Kassel, with Nassau, some former Hesse-Darmstadt districts, and Frankfurt, was made part of the Prussian province of Hesse-Nassau. This in 1944 was split into Nassau and Electoral Hesse (Kurhessen); but on Sept. 19, 1945, these provinces were united with the part of Hesse-Darmstadt east of the Rhine to form Greater Hesse, later called simply Hessen.