His 1944 work, "Public Opinion and the Last Peace", is an analysis of the relationship between public opinion and policy in regards to the making of the Treaty of Versailles and its revision during the Interwar years. McCallum defended David Lloyd George against those who had attacked him for his conduct during the 1918 election and for his policies at the Paris Peace Conference. He also defended the Versailles Treaty against the criticism of John Maynard Keynes in "The Economic Consequences of the Peace". Max Beloff called "Public Opinion and the Last Peace" "scholarly and penetrating", adding that it was "one of the most important, as it is certainly one of the most brilliant and courageous, contributions to the literature of the subject".