[1][2] Between 1966 and 1970 he was president of the Italian Historical Institute for the Middle Ages [it] (ISIME) in Rome.
[1][2] Arnaldi was the author of several historical essays, and his researches mainly focused on the role of Italy and of the papacy in shaping the medieval civilization.
[1] He also directed several collective works such as the Encyclopaedia of the Popes and the Enciclopedia Fridericiana for Treccani.
[1] He collaborated with RAI for the documentary series La straordinaria storia dell’Italia.
[1] His last book was L’Italia e i suoi invasori (2002), which in 2005 was translated in English and published by Harvard University Press with the title Italy and Its Invaders.