After his MA Claessen became a teacher of social geography in Sint Adelbert College (1956–1970), a period during which he prepared his PhD thesis under A. Köbben's supervision.
The concept of the early state introduced by Henri J. M. Claessen and Peter Skalník appears to have been the last among the great epoch-making political-anthropological theories of the 1960s and 1970s.
[6] Claessen's thesis, Of Princes and Peoples, a comparative study of the political organization of five Early States (Tahiti, Tonga, Dahomey, Buganda and the Realm of the Incas) where the emphasis came to be the central political organisation, which has remained an important theme in much of Claessen's work, lay at the basis of The Early State (1978) which he edited with Peter Skalník.
With Renée Hagesteijn and Pieter van de Velde he edited a special issue of Social Evolution & History under the title Thirty Years of Early State Research (2008).
[8] Claessen's school developed a "Complex Interaction Model" (CIM) to explain the evolution of sociopolitical organization and early state formation.