 Complete the following phrase:  At wars end he renewed his studies, focusing on Eastern European history successively at Jena, Frankfurt am Main and Berlin, where he received his PhD, with a dissertation on the court and administration of Russia from the 15th to the 17th century, in 1924. He undertook his Habilitationsschrift at the University of Hamburg, under Richard Salomon, from 1926 to 1931, and Frankfurt University from 1932 to 1933, on the International Relations of the Soviet Union and the 