He wrote several major works on Asiatic and Oriental civilizations, with his two most important works being History of the Crusades (1934–1936) and The Empire of the Steppes, a History of Central Asia (1939), both of which were considered standard references on the subject.
Ia menulis beberapa karya besar tentang peradaban Asiatik dan Oriental, dengan dua karya paling berpengaruh meliputi Sejarah Perang Salib (1934-1936) dan Kekaisaran Wilayah Stepa, sebuah Sejarah Asia Tengah (1939), yang keduanya dijadikan rujukan standar pada subyek tersebut.