English: Three chief retainers of the Owari branch of the ruling Tokugawa clan were executed in the Ninomaru Palace of Nagoya Castle in 1868, what became known as the Aomatsuba Incident. Early in the Showa era, around 1926, a monument was erected at the execution site. The exact site is unknown although it is thought to have taken place approximately 100 metres south of the current site of the monument. The stone stelae was re-erected after the original one disappeared.