 Upon the death of Valentinian in 375, Gratian's brother Valentinian II was declared emperor by his father's soldiers.In 378, Gratian's generals won a decisive victory over the Lentienses, a branch of the Alamanni, at the Battle of Argentovaria.Gratian subsequently led a campaign across the Rhine, the last emperor to do so, and attacked the Lentienses, forcing the tribe to surrender.