[5] Arup was also the editor of Historisk Tidsskrift from 1917 to 1924, involved in editing Danish medieval sources from 1931 to 1937, and for some years collaborator to the pioneering Danish-Swedish periodical Scandia.
His breakthrough came with his dissertation Studier i engelsk og tysk Handels Historie (1907), a pioneering work stressing the geographic and economic, instead of the national or ethnic, background behind the development of trade.
In addition to this work, he notably wrote on economic history in Schleswig-Holstein, on medieval economy, and on 18th Century foreign policy - besides producing a quick and popular biography of his inspirational figure Hørup (1941).
What is innovative in this work is Arup's coverage of his country's evolution mainly from the angle of material development, of agriculture, of trade, and to some extent of public health.
Arup concentrated on period documents rather than chronicles or other secondary narratives, and more generally introduces strict demands regarding source criticism.
More relevant today perhaps, is the critique of Arup as lacking historical feel and holding anachronistic views (such as his attempt to find “parliamentarian” traces in medieval government).
Nonetheless, Danmarks Historie is in its own country still regarded as the most consistent break with traditional historical writing, and has offered much inspiration to later social-liberal and Marxist historians.