One year later, he began his doctoral thesis on the role of American missionaries in Bulgaria and their impact on the process of the Bulgarian National Revival.
From 1931 to 1934, Clarke worked in the Bulgarian archives and libraries, visiting numerous settlements related to his research.
Clarke, as well as his missionary predecessors who were direct observers of the Balkan historical scene, adhered to the perception of the Bulgarian identity of the Macedonian Slavs.
Without denying the right of this population to self-identity, he defined the concept on the "Macedonian language" as a myth, arguing with his American counterpart Horace Lunt.
[2] He is the author of a collection of studies on Bulgarian history, called The pen and the sword, edited by Dennis P.