Dueling didn't die out because of sudden opposition to the practice. In fact, there have been calls to ban dueling dating back centuries. Christian leaders disliked dueling because it clearly violated one of the commandments. They also were against it because "legal ordeal" dueling took power away from church officials, who would have preferred to judge such cases themselves. Church opposition to dueling continued from the middle ages until dueling's eventual demise. Kings and military leaders opposed dueling at times because it cost the noble classes so many young men who might otherwise have filled the officer ranks in the military.
In the 1800s, politicians, judges and writers were very vocal in their desire to see dueling banned. Mark Twain was against dueling (Holland, pg. 214), and both George Washington and Benjamin Franklin found dueling a waste of human life [ref]. Many states passed laws against dueling, but for many years juries refused to find anyone guilty of the crime.
The ultimate demise of dueling was due to a complex set of cultural factors. It had survived for centuries as something carried out by noble men to help keep themselves distinct from the lower classes. Once dueling had spread to every stratum of society, it no longer served this function. At that point, the destructive nature of dueling began to have an impact on public opinion. Also, some historians speculate that the great wars of the 19th and 20th centuries exposed people to the horrors of combat while simultaneously killing off a large portion of the younger generation. The Civil War in the United States and World War I in Europe mark rough points at which dueling began to decline in the respective cultures.
Today, dueling still exists, but it has taken less bloody forms. In the purest sense, one-on-one contests such as boxing and wrestling capture the spirit of dueling, while fencing as a sport descends directly from duels. Almost any head-to-head showdown guided by very specific rules of etiquette can be considered a modern-day duel and may show up anywhere -- at the poker table, in the corporate boardroom, on the tennis court or in video games.
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