It wasn’t particularly funny. The actual casus belli was the loss of the British trading ship the Rebecca to Spanish coast guards. In the 1730s the British and Spanish had been variously raiding each others trading ships. Political opponents of Prime Minister Robert Walpole used the sensational claims by Captain Robert Jenkins, that he had lost both his ship, the Rebecca, and his ear, to Spanish coast guards, to pressure the Prime Minister. Walpole declared war in 1739.

Ref: Dictionary of Wars: George Childs Kohn