Philip's eldest son, William IV, received Hesse-Kassel, which comprised about half the area of the Landgraviate of Hesse, including the capital, Kassel.

William's brothers received Hesse-Marburg and Hesse-Rheinfels, but their lines died out within a generation, and the territories then reverted to Hesse-Kassel and to the Landgraviate of Hesse-Darmstadt.

In 1801 he lost his possessions on the left bank of the Rhine, but in 1803 he was compensated for these losses with some former French territory around Mainz, and at the same time he was raised to the dignity of Prince-elector (Kurfürst) William I, a title he retained even after the dissolution of the Holy Roman Empire.

A treaty concluded by him with the Coalition (2 December) stipulated that he was to receive back all his former territories, or their equivalent, and at the same time restored the ancient constitution of his country.

[3] William I had marked his restoration by abolishing with a stroke of the pen all the reforms introduced under the French regime, repudiating the Westphalian debt and declaring null and void the sale of the crown domains.

He was arbitrary and avaricious, like his father, and moreover shocked public sentiment by his treatment of his wife, a popular Prussian princess, and his relations with his mistress, one Emilie Ortlöpp, whom he created Countess of Reichenbach-Lessonitz and loaded with wealth.

[3] The July Revolution in Paris gave the signal for disturbances; William II was forced to summon the Estates, and on 6 January 1831, a constitution on the ordinary Liberal basis[clarification needed] was signed.

All the efforts of William II and his minister were directed to nullifying the constitutional controls vested in the Diet; and the Opposition was fought by manipulating the elections, packing the judicial bench, and a vexatious and petty persecution of political "suspects", and this policy continued after the retirement of Hassenpflug in 1837.

[3] The consequences emerged in the revolutionary year 1848 in a general manifestation of public discontent; and Frederick William, who had become Elector on his father's death (20 November 1847), was forced to dismiss his reactionary ministry and to agree to a comprehensive programme of democratic reform.

Hassenpflug persuaded the Elector to leave Kassel secretly with him, and on 15 October, appealed for aid to the reconstituted federal diet, which willingly passed a decree of "intervention".

[3] This was a direct challenge to Prussia, which under conventions with the Elector had the right to use the military roads through Hesse that were her sole means of communication with her exclaves in the Rhine provinces.

In 1855, however, Hassenpflug who had returned with the Elector was dismissed; and five years later, after a period of growing agitation, a new constitution was granted with the consent of the federal diet (30 May 1860).