Year 423 (CDXXIII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Marinianus and Asclepiodotus (or, less frequently, year 1176 Ab urbe condita). The denomination 423 for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.
Winter – Emperor Theodosius II refuses to recognize Joannes as emperor, and prepares for war. He mobilizes an expeditionary force under command of Ardaburius, and his son Flavius Aspar.