The Ardennes is a region of extensive forests, rough terrain, rolling hills and ridges formed by the geological features of the Ardennes mountain range and the Moselle and Meuse River basins. Geologically, the range is a western extension of the Eifel and both were raised during the Givetian stage of the Devonian (419.2 ± 3.2 to about 358 million years ago) as were several other named ranges of the same greater range.
Primarily in Belgium and Luxembourg, but stretching into Germany and France (lending its name to the Ardennes department and the Alsace-Champagne-Ardenne-Lorraine region), and geologically into the Eifel—the eastern extension of the Ardennes Forest into Bitburg-Prüm, Germany, most of the Ardennes proper consists of southeastern Wallonia, the southern and more rural part of the Kingdom of Belgium (away from the coastal plain but encompassing over half of the kingdom’s total area). The eastern part of the Ardennes forms the northernmost third of the Grand Duchy of Luxembourg, also called “Oesling” (Luxembourgish: Éislek), and on the southeast the Eifel region continues into Rhineland-Palatinate (German state).
The trees and rivers of the Ardennes provided the underlying charcoal industry assets that enabled the great industrial period of Wallonia in the 18th and 19th centuries, when it was arguably the second great industrial region of the world, after England. The greater region maintained an industrial eminence into the 20th century after coal replaced charcoal in metallurgy.
Allied generals in World War II felt the region was impenetrable to massed vehicular traffic and especially armor, so the area was effectively “all but undefended” during the war, leading to the German Army twice using the region as an invasion route into Northern France and Southern Belgium via Luxembourg in the Battle of France and the later Battle of the Bulge.