The answer to this question can be found early in the first chapter of the book. My copy is on Kindle so it has no page numbers, but a search of Amazon’s copy shows that the relevant passage is on p. 13 of the paperback edition.
William Dodd had done his doctoral research at the University of Leipzig in Germany. While there, he had observed German attitudes closely. He had come to believe that they were excessively militaristic. On p. 12, Erik Larson says that Dodd wrote that
There was too much war spirit everywhere.
Dodd only stayed in Germany two years. However, his time there gave him clear opinions about the subject of Germany when World War I started. Larson says that Dodd believed that
Germany alone was responsible for starting the war…
Larson says that Dodd believed that the Germans had started the war because that was what the industrial elites and the aristocratic “Junkers” wanted. He believed that these Junkers were similar to the slave-owning elites of the antebellum United States. Even so, Dodd did not back American participation in the war. He feared that American industrialists and military commanders were getting to be too much like those in Germany.
Dodd, a historian who was working on a history of the antebellum South and the Civl War when he became U.S. ambassador to Germany, firmly believed Germany alone had started World War I. He likened the yearning of Germany's Junkers, aristocrats and industrialists for war to those of upperclass warmongering Southerners. His experiences in Germany before World War I also convinced him that Germany was overly enamored of the military. He loved the country but not its militarism.
Although Dodd returned to Germany as the U.S. ambassador in 1933 determined to have an open mind about the new Nazi regime, he and his family, grew more and more disillusioned and even terrified by it.
At a bizarre lunch at the Goering's estate later in his stay, after which the guests were touring the grounds, the talk turned enthusiastically to a new book about the German navy in World War I. Dodd responded that "If people knew the truth of history there would never be another great war."