A committed monarchist and, till the First World War, well networked internationally with fellow medievalists, his personal diaries, which have been extensively researched and analysed posthumously, provide insights which are enlightening about the times through which he lived, and quite possibly constitute a more important contribution to European historiography than any of his published articles and books on the Medieval centuries. The war led him to break off his international contacts, most importantly with French and Belgian medieval scholars. He was never reconciled to the post-imperial "Weimar Republic" (as Adolf Hitler scathingly termed the democratic regime that operated in Germany between 1919 and 1933, thereby popularising a term that was quickly adopted by historians during the 1930s). Cartellieri campaigned against the "burdensome and humiliating" Versailles peace treaty, even though – unlike many German historians of the time – he never retreated into a narrowly nationalist historical vision. After 1933 the foreign policy successes of the Hitler government impressed him, but he resisted or ignored any pressures to become a propagandist for a National Socialist government strategy that was more and more obviously based on violence, both domestically and on the world stage.