This is a story of family rows and intrigue that centres on three generations of the family that ruled Hesse-Kassel – William VIII, his son Frederick II and his grandson William IX. The history books don't go into their personal relationships but you can read between the lines.
When the hereditary prince of Hesse-Cassel, Frederick (later to be Frederick II) , converted to Roman Catholicism in 1747, and, incidentally, abandonned his family, his father, the reigning landgrave William VIII decided to do what he could to limit his son's future realm. He therefore made the county of Hanau-Münzenberg, which had been part of Hesse-Cassel since 1736, a secundogeniture of Hesse-Cassel, transferring it to his grandson William. A secundogeniture (from Latin:secundus "following, second," and genitus"born") was a dependent territory given to a younger son of a princely house and his descendants, creating a cadet branch. This was a special form of inheritance in which the second and younger son received more possessions and prestige than he would have normally under the rules of primogeniture. It avoided the division of the estate and gave younger branches a stake in the stability of the house.
As count William was underage, his mother the landgravine, princess Mary of Great Britain, ruled as his legal guardian. His childhood was mostly spent in Denmark. After his accession to the throne of Hesse-Cassel in 1760, landgrave Frederick II repeatedly tried to re-unite Hesse-Hanau with Hesse-Cassel, but his efforts failed due to the resistance of Great Britain and the Protestant estates. As further protection, troops from Hanover were garrisoned in Hanau. When William came of age in 1764 he took over the government of the county. Two years later he signed an agreement with Great Britain to supply troops when necessary. Both he and his father waived the taxes of families of men who would serve in America. Had father and son been reconciled by then?
At the death of Frederick II in 1785, William became landgrave of Hesse-Cassel. The government of Hesse-Hanau remained in general separate from Hesse-Cassel. Cabinet and war office were, however, merged with those in Hesse-Cassel, and the court of appeal of Cassel got jurisdiction over Hanau in 1792. Until then Hesse-Hanau was ruled as an independent state, undergoing extensive modernizations with the erection of significant buildings in the capital of Hanau. Money for this came from the subsidies the reigning count received from his uncle, king George III of Great Britain. In return, Hesse-Hanau made available a contingent of 2,400 soldiers for the use of the British Crown in the American Revolutionary War.