Between Harvard and Yale he served in the United States Army, including a tour of duty in Vietnam in 1969 as a rifle platoon commander with the 173rd Airborne Brigade.
As a historian, Knox has specialized in the political, military and diplomatic history of Europe in the late 19th and 20th centuries, focusing on the two world wars and the emergence of dictatorship in the 1920s and 1930s.
[4] Knox's most recent work, To the Threshold of Power, is a comparative study of the rise of Benito Mussolini and Fascism in Italy with that of Adolf Hitler and National Socialism in Germany.
On the other hand, he argues that the leadership of the Social Democratic Party of Germany, from Friedrich Ebert to Otto Braun, was the strongest pillar of German democracy under the Weimar Republic.
He sees the failure of democracy in Italy and Germany as ultimately caused by the desire of the armies in both countries for an authoritarian regime that would suppress the parties of the left and allow rearmament, and a foreign policy that would reverse the verdicts of 1918.