Born in
Kassel, he was the seventh son of
Charles I, Landgrave of Hesse-Kassel and
Maria Amalia of Courland. After his elder brother
Frederick became
King of Sweden in 1720 and his father died in 1730, he became de facto ruler of Hesse-Kassel. He officially became landgrave after his brother's death on 25 March 1751. Five years later, the
Seven Years' War began and William joined with the
Prussian and
British forces.
Hesse-Kassel became an important battlefield and was occupied by
France on several occasions. He had a deep, personal friendship with
Frederick the Great,
King of Prussia, and
Holy Roman Emperor Charles VII. His second son and successor, Frederick, became a Catholic, which led to restrictions on
Catholicism in the
Calvinist landgraviate and the transfer of the
Principality of Hanau to his
Protestant grandson
William. During his reign, William started building Schloss Wilhelmsthal in
Calden and collected paintings, including works by
Rembrandt. File:Schloss Wilhelmsthal - Wilhelmsthal.JPG|Schloss Wilhelmsthal ==Family==