The childless Theodosius died in a riding accident on 28 July 450. His sister Pulcheria chose an elderly military commander Marcian as her husband without consulting with Valentinian. She allegedly acted in concern with the all-powerful Aspar who had been Marcian's superior in the army. Marcian was proclaimed emperor in Constantinople in late August. On learning of Attila's plan about a military campaign in the west he stopped tribute payments to the Huns. Attila launched a massive incursion into Gaul at the head of a mixed army of Huns and subject peoples. Aetius assembled Roman, Visigothic and Burgundian troops and engaged the enemy at the Catalaunian fields in June 451. Although the battle was inconclusive, Attila withdrew from Gaul. Next year he invaded Italy, but supply problems and an epidemic forced him to again withdraw. He died unexpectedly of bleeding in 453. In a year, the Hunnic Empire collapsed due to a civil war between his sons and a revolt of the subject peoples. With the Hunnic threat vanishing, Valentinian got rid of the domineering Aetius with the assistance of his eunuch courtier Heraclius who murder the general in September 454. Aetius' death was revenged by his two retainers who assassinated Valentinian on 16 March 455.