269 - Battle of Lake Benacus
Following Emperor Gallienus' assassination the previous year, Claudius II becomes the Emperor of Rome and defeats the Alemanni in the Battle of Lake Benacus (modern day Lake Garda).
Claudius fails to restore the original frontier which at its high water mark - the 'Upper Raetian Limes' - extended between c.50km and c.100km east of the Rhine, and up to 60km north of the Danube, until becoming the Danube c.30km west of modern day Augsburg.
The new frontier is pulled back to the Donau-Iller-Rhine-Limes, and follows the Rhine as far west as Lake Constance, then the river Iller north until it joins the Danube at the modern day city of Ulm, and then along the Danube river, thus leaving a large Alemannic enclave (shaded) wedged into the Roman Empire's northern border.
Image credit: http://en.wikipedia.org/wiki/File:Limes2.png