Why was Louis XVI arrested?

Louis was officially arrested on 13 August 1792 and sent to the Temple, an ancient Paris fortress used as a prison. On 21 September, 1792 the National Convention declared France to be a republic and abolished the monarchy.

He was arrested because he tried to flee the country with his family and requested the help of other European monarchs to overcome this revolution and restore the monarchy in France. This was seen as treason, and this is what he was tried for.