Engelmann's historical perspective was especially evident in "The European Empire: From Charlemagne to the Common Market" (1962). The geographic similarity between Charlemagne's Empire and that of the Common Market almost twelve-hundred years later was attributed to similar phases of migration patterns. Like the outward expansion of Charlemagne's forces, a similar pattern was noticed in the Common Market. Even Charlemagne's capital, Aachen, is a mere 75 miles from NATO headquarters in Brussels. Matthew Omolesky, in his 2009 article about the European Union, "Between Rome and Byzantium," cites Engelmann's observation about the geographic similarity between the Common Market and Charlemagne's Empire too.