TheAgri Decumates orDecumates Agri ("Decumatian Fields") were a region of theRoman Empire's provinces ofGermania Superior andRaetia, covering theBlack Forest,Swabian Jura, andFranconian Jura areas between theRhine,Main, andDanube rivers, in present southwesternGermany, including presentFrankfurt,Stuttgart,Freiburg im Breisgau, andWeißenburg in Bayern.
The only ancient reference to the name comes fromTacitus' bookGermania (chapter 29).[1][2] However, the later geographerClaudius Ptolemy does mention "the desert of the Helvetians" in this area.[3]
The meaning ofDecumates is lost and has been the subject of much contention. According to the English ClassicistMichael Grant, the word probably refers to an ancient Celtic term[4] indicating the political division of the area into "ten cantons." Another theory is that the term implies that atithe was paid by residents living in this country.[5]
According to Tacitus, the region was originally populated by theCeltic tribe of theHelvetii, but soonGermanic and Gaulish settlers arrived. Tacitus writes that:
I should not reckon among the Germanic tribes the cultivators of the tithe-lands [agri decumates], although they are settled on the further side of the Rhine and Danube. Reckless adventurers from Gaul, emboldened by want, occupied this land of questionable ownership. After a while, our frontier having been advanced, and our military positions pushed forward, it was regarded as a remote nook of our empire and a part of a Roman province.[2]
Under theFlavian and later emperors, Romans took control and settled the region.[2] They built a road network for military communications and movements, and improved protection from invading tribes using the region to penetrate intoRoman Gaul. Frontier fortifications (limes) were constructed along a line running Rheinbrohl—Arnsburg—Inheiden—Schierenhof—Gunzenhausen—Pförring (Limes Germanicus).
The larger Roman settlements wereSumelocenna (Rottenburg am Neckar),Civitas Aurelia Aquensis (Baden-Baden),Lopodunum (Ladenburg) andArae Flaviae (Rottweil).
Romans controlled the Agri Decumates region until the mid-3rd century, when the emperorGallienus (259–260) evacuated it before the invadingAlemanni and the secession of much of theWestern Roman Empire under the "usurper and ruler"Postumus.[6]
The emperorAurelian (AD 270–275) may have had the region briefly reoccupied during the Roman resurgence of the late 3rd century under the so-called "military" emperors. Even if this did occur, re-establishment of Roman rule was brief. EmperorProbus definitely recaptured the area during his invasion of Germania. This presence lasted until the 290s or 300s, when the Roman military seems to have definitively abandoned the region. By about 310 the Alemanni were becoming the dominant population, although scattered Roman settlements seem to have persisted, and continued to trade with the empire, until about the mid 4th century. Germanic peoples have continuously inhabited the region since then.[1] However, Roman settlements were not immediately abandoned. There is evidence the Roman way of life continued well into the 5th century, much as Roman patterns continued in neighbouring Gaul long after the Western Roman Empire's collapse.
J. G. F. Hind has suggested[7] the former Roman inhabitants of theAgri Decumates were to be found from the later 3rd to the 5th centuries in theDecem Pagi—also "ten cantons"—having transferred west of the Rhine, to the region between the Rhine and theSaar, betweenMainz andMetz.