He wrote articles on international politics and current affairs for publications such as the New York Times, Century, and the New Republic.
In 1921, the New Republic announced that Bagger was editing a Hungarian-language magazine, New Age, which declared itself "uncompromisingly opposed to any idea of americanization [sic] involving kneading the immigrant into static moulds."
Author of multiple biographies, his Eminent Europeans: Studies in Continental Reality was widely reviewed when it was released in 1922.
He followed for a time a journalist's career in America writing for The Nation, The New Republic, The Century Magazine and The Atlantic Monthly.
Bagger eventually returned to Europe, in 1924, with a commission to write the life of the late Emperor Franz Joseph I of Austria.