[2] When Willem IV died, almost all of the inheritance went to Gerard II of Horne, oldest son out of his first marriage.

His interests were promoted by his uncle Dirk van Horne, oldest son of Gerard I and Irmgard of Kleve.

He was opposed by Gerard II's sister Johanna van Horne, married to Gijsbrecht III of Abcoude.

[6] The Lordship of Weert, the Guardianship of Thorn Abbey, the village Beket, and Munnikenland were fiefs of the Duchy of Guelders.

Willem was only a half brother of the deceased Gerard II, while Johanna was his full sister.

However, already on 20 December 1345 Dirk van Horne was granted the fief Altena by emperor Louis the Bavarian (reign 1314–1347).

On 17 March she then made a surprising move, by promising to grant Altena to Gijsbrecht III of Abcoude within one month of her expected arrival in Holland.

It was a chaotic time, and in these circumstance Knight Willem van Rijswijk traveled to The Hague on 26 September 1347.

On the very same day, the Land of Altena was granted to the 10 year old Willem van Horne, as if he had been present.

[9] During the Hook and Cod Wars Willem V van Horne sided with his father in law John IV of Arkel.

[12] When war broke out between Holland and Utrecht in November 1355, Arkel paid money to the count, in order not to have to fight his half-brother the bishop.

[13] On 3 April 1357 Willem's younger brother Dirk Loef van Horne was granted the Land of Altena by Count William V of Holland.