The theme of this work is best summarized in a sentence on page 151, "No two generations of barbarians experienced exactly the same Roman Empire , nor did Romans discover the same barbarians living along their borders as had their fore-fathers." That is, there was constant change in the nature of each of the two "sides," rooted in perception of "the other," cultural, social, and political changes within the Empire and outside of it, and more. 

Prof. Thomas (Emory) manages a very thoughful, complex, and yet readable trek through the changing ways in which Romans and "barbarians" perceived and interacted with each other across more than five centuries, with a side-long glance at how later scholars, historians, nationalists, artists, and others interpreted those relationships in the light of their own cultural perceptions, trying to build "models" that taught "lessons," a phenomenon hardly dead among modern politicians and pundits, who usually get it wrong. 

A very good read for any student interested in the Romans or the barbarians.