In 1998, he published a comprehensive study on the history of Belgian foreign policy ("Belgium and its foreign policy 1830–1990", in Dutch and partly in French). The latest revised edition, released in September 2014, pursues this history until 2014 (published only in Dutch). Two other studies on Belgian foreign policy deal with Dutch-Belgian bilateral relations since 1945 ("Nederland-België. De Belgisch-Nederlandse betrekkingen vanaf 1940", Boom, 2011, with Duco Hellema and Bart Stol) and with the history of the Ministry of Foreign Affairs: "Les Affaires étrangères au service de l’Etat belge, de 1830 à nos jours" (Mardaga, 2014) and, in Dutch, "Buitenlandse Zaken in België. Geschiedenis van een ministerie, zijn diplomaten en zijn consuls van 1830 tot vandaag" (Lannoo, 2014), with Vincent Dujardin and late Claude Roosens.