The Alemanni or Alamanni were a confederation of Germanic tribes on the Upper Rhine River during the 1st millennium. They are first mentioned by Cassius Dio in the context of the campaign of Roman emperor Caracalla of 213 AD, the Alemanni captured the Agri Decumates in 260 AD, and later expanded into present-day Alsace and northern Switzerland, leading to the establishment of the Old High German language in those regions, which by the 8th century were collectively referred to as Alamannia.