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Timothy P. Maga, "Diplomat among Kings: John Cudahy and Leopold III." Wisconsin Magazine of History 67.2 (1983): 82-98 online.
persee.fr
P.H. Laurent, "Change and Continuity in a Diplomacy: the First Americans in Brussels." Revue belge de Philologie et d'Histoire 45.2 (1967): 388-407 online.