Sending an ear in the mail is never a good idea
Known by a much more reasonable name among the Spanish, this is a war that lasted for nine years after starting because of a single severed ear. This war was a part of the war for Austrian Succession, a long and bloody conflict that is interesting but not nearly as weird as this little corner of it.
Robert Jenkins was a British Captain and acknowledged smuggler. Operating in the British West Indies and out of England itself, he was a master mariner and a valued member of the British navy. The incident happened as he was commanding the brig Rebecca and sailing out of the West Indies.
Robert Jenkins's ship was stopped by the Spanish under suspicion that he was smuggling. Since he was, they found him guilty in quick order. He was tied to the mast and had his ear cut off by the Spanish, who told him "same will happen to him (the king) if caught doing the same".
They gave him his ear, for some reason, and sent him on his way. After returning to England he found little sympathy with only a short news article being written about it. Angry with the Spanish, Jenkins took his story to the House of Commons, where he told them of the incident and showed them the carefully preserved ear. As many lawyers and other presenters will tell you, props can be a big help. This brought the British on board, as well as flaming jealous and anger about their lack of involvement within the Spanish colonial trade.
Jenkins was given command of a new vessel in the navy, and went on his way.
This story was looked at with skepticism even in its day. In the late 1880s, the founder of the Navy Records society researched the incident and found evidence that the entire incident actually happened, making this another incident of a small matter turning into a war.