Valentinian I (Latin: Valentinianus; 321 17 November 375), sometimes called Valentinian the Great, was Roman emperor from 364 to 375. Upon becoming emperor, he made his brother Valens his co-emperor, giving him rule of the eastern provinces. Valentinian retained the west.

During his reign, Valentinian fought successfully against the Alamanni, Quadi, and Sarmatians. Most notable was his victory over the Alamanni in 367 at the Battle of Solicinium. His general Count Theodosius defeated a revolt in Africa and the Great Conspiracy, a coordinated assault on Roman Britain by Picts, Scots, and Saxons. Valentinian was also the last emperor to conduct campaigns across both the Rhine and Danube rivers. Valentinian rebuilt and improved the fortifications along the frontiers, even building fortresses in enemy territory.

He founded the Valentinianic dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire.

The Alemanni (also Alamanni; ) were a confederation of Germanic tribes on the Upper Rhine River. First mentioned by Cassius Dio in the context of the campaign of Caracalla of 213, the Alemanni captured the Agri Decumates in 260, and later expanded into present-day Alsace, and northern Switzerland, leading to the establishment of the Old High German language in those regions, by the eighth century named Alamannia.In 496, the Alemanni were conquered by Frankish leader Clovis and incorporated into his dominions. Mentioned as still pagan allies of the Christian Franks, the Alemanni were gradually Christianized during the seventh century. The Lex Alamannorum is a record of their customary law during this period. Until the eighth century, Frankish suzerainty over Alemannia was mostly nominal. After an uprising by Theudebald, Duke of Alamannia, though, Carloman executed the Alamannic nobility and installed Frankish dukes.

During the later and weaker years of the Carolingian Empire, the Alemannic counts became almost independent, and a struggle for supremacy took place between them and the Bishopric of Constance. The chief family in Alamannia was that of the counts of Raetia Curiensis, who were sometimes called margraves, and one of whom, Burchard II, established the Duchy of Swabia, which was recognized by Henry the Fowler in 919 and became a stem duchy of the Holy Roman Empire.

The area settled by the Alemanni corresponds roughly to the area where Alemannic German dialects remain spoken, including German Swabia and Baden, French Alsace, German-speaking Switzerland, Liechtenstein and Austrian Vorarlberg.

The French language name of Germany, Allemagne, is derived from their name, from Old French aleman(t), from French loaned into a number of other languages, including Middle English which commonly used the term Almains for Germans. Likewise, the Arabic name for Germany is ألمانيا (Almania), the Turkish is Almanya, the Spanish is Alemania, the Portuguese is Alemanha, Welsh is Yr Almaen and Persian is آلمان (Alman).