Schubert, who had been working for the Historical Commission of the Bavarian Academy of Sciences since 1952, began a major study of the older German Reichstag in the image of journalism between 1495 and 1648 after the printing of his dissertation.
Only one year later, this time inspired by Carl Dietrich Erdmann, he was appointed Professor of Medieval and Modern History at the Christian-Albrechts-Universität zu Kiel.
Schubert belonged to the rediscoverers of the older German Reichstag as a weighty institution in the European environment and, incidentally, also of the work of the Calvinist state theorist Johannes Althusius.
Like Franz Schnabel, Schubert was a representative of the German new humanism, which had a strong impact in Munich in the immediate post-war period.
Schubert not only had a Western and liberal understanding of history, but also made a significant contribution to placing the older German constitutional institutions at the centre of the common European intellectual tradition throughout the early modern period.