An influential figure, he worked on propaganda in World War I, and, when the war was over, he was involved in the drafting of the Versailles Treaty especially regarding Danzig. He effectively sponsored Arnold J. Toynbee for appointment in 1924 to Chatham House. He also gathered materials on the diplomatic history of the origins of World War I, as an official production of the British government, and contributed to it, though the main editor was Harold Temperley. Historian Anna Cienciala attributes to Headlam and Sidney Edward Mezes, an academic and advisor to Woodrow Wilson and Executive Director of the Inquiry group, the 1919 proposal to make Danzig a free city.