With the decline of the Roman Empire, Alsace became the territory of the Germanic Alemanni. The Alemanni were agricultural people, and their Germanic language formed the basis of modern-day dialects spoken along the Upper Rhine (Alsatian, Alemannian, Swabian, Swiss). Clovis and the Franks defeated the Alemanni during the 5th century AD, culminating with the Battle of Tolbiac, and Alsace became part of the Kingdom of Austrasia. Under Clovis' Merovingian successors the inhabitants were Christianized. Alsace remained under Frankish control until the Frankish realm, following the Oaths of Strasbourg of 842, was formally dissolved in 843 at the Treaty of Verdun; the grandsons of Charlemagne divided the realm into three parts. Alsace formed part of the Middle Francia, which was ruled by the youngest grandson Lothar I. Lothar died early in 855 and his realm was divided into three parts. The part known as Lotharingia, or Lorraine, was given to Lothar's son. The rest was shared between Lothar's brothers Charles the Bald (ruler of the West Frankish realm) and Louis the German (ruler of the East Frankish realm). The Kingdom of Lotharingia was short-lived, however, becoming the stem duchy of Lorraine in Eastern Francia after the Treaty of Ribemont in 880. Alsace was united with the other Alemanni east of the Rhine into the stem duchy of Swabia.

Which three territories make up the Upper Rhine?