Baden is the area of present-day southwestern Germany. Although its borders have changed over time, when it was part of the German empire from 1871-1918 (These years are important to family history research as the Family History Library catalogs German records according to the boundaries as they existed at this time.), it was bordered on the south by the Rhine, on the northwest by The Palatinate, on the north by Hesse-Darmstadt and Bavaria, on the east by Württemberg, and the southeast by Hohenzollern and Lake Constance. Click here to see a map of Baden.
The earliest inhabitants of the area, from approximately the 4th century BC, were Celtic tribes. Although these tribes left little in the way of influence, one of them gave their name to a by-name of Switzerland, the Helvetii. By the 2nd century BC, Germanic tribes moved into the area, as did the Romans, who included the area of Baden in their province of Germania Superior. The name Baden, however, does not appear at this time. Conflict between the Romans and Germanic tribes raged for centuries, causing the Romans to build a defensive barrier at the outermost limits of the Empire. This construction, known as the Limes Germanicus (Latin for ‘limit’), stretched from the Danube to the Rhine and included almost all of Baden. The Germans of this time were loosely confederated groups at best, and sometimes totally independent of each other. One such confederacy was that of the Allemanni, which has given its name to Germany in French, Spanish, and Portuguese. However, it seems that the people called themselves Suevi or Suebi (cf. Schwaben). Ultimately, the Allemanni thoroughly germanized the area and only place names from the Celts and Romans survive.
Here is a link to describe the history, population and geography of Baden: