[2] From 1946, when she took her doctor's degree with the thesis Političko i društveno uređenje Slavonije za Arpadovića (Political and Social Organization of Slavonia under the Árpád dynasty),[2][1] she spent several decades engaged in researching Croatian medieval history.
Nada Klaić gathered the results of her extensive analytical investigations, published first in journals and proceedings, and monographs and surveys of Croatian history.
[2] She provided a comprehensive and original concept of the early medieval development of the Croatian lands in the book History of the Croats in the Early Middle Ages (1971), while she collected her writings about numerous problems of the later period in the book History of the Croats in the High Middle Ages (1976).
[2] Her posthumously published books are Medieval Bosnia: Political Status of Bosnian Rulers before the Coronation of Tvrtko in 1377 (1989) and History of the Croats in the Middle Ages (1990).
Some of her achievements are the innovative and modern approach to Croatian history (especially for the Middle Ages), which helped release it from the romantic nationalism of the 19th century,[2] and the revaluation of older historical sources.
[6] The German and Austrian scholars H. Kunstmann, J. Herrmann, R. Werner and O. Kronsteiner loose considerations on Slavs were especially influential on her viewpoints.
[15] Miroslav Brandt claimed that Klaić, even though she was usually described as iconoclast,[8] is a protégé of socialist Yugoslavia in her approach to Croatian historiography.