Admiral John Byng was executed by firing squad on March 14, 1757, on the quarterdeck of HMS Monarch at Portsmouth for “failing to do his utmost” to relieve Minorca from the French during the Seven Years’ War, becoming the first and last British admiral shot for cowardice, a controversial event popularized by Voltaire’s famous quote about encouraging the others. He was found guilty by court-martial, as the government used him as a scapegoat for broader strategic failures, and King George II refused a pardon.
Read a very good article about Byng on Wikipedia.