He returned to Berlin at the beginning of February 1806, accepting an invitation to take a job as an assistant to the book dealer and publisher Heinrich Frölich whose business, since its establishment in 1798, had thrived: Frölich had built up excellent literary and commercial connections. However, just six weeks after Duncker had joined him, on 11 March 1806 Frölich died suddenly, and Carl Duncker found himself running the firm. Duncker was confident that he would be able to sustain the business, but Frölich's widow was vehemently of the opinion that he was too young and inexperienced to do so. They called in the author and book dealer Friedrich Nicolai, a venerated elder of the Berlin book world, to arbitrate. Nicolai came down in support of Duncker who was left in post, to manage the business through what proved to be an exceptionally difficult couple of years for those seeking to live through commerce. Berlin was occupied by French troops between October 1806 and December 1808, while intensified fighting persisted in Silesia and East Prussia, where the king had been obliged to move with his court and government following military defeat in 1806. International trade was also inhibited by the blockade of the Prussian ports imposed by the British navy in response to Emperor Napoleon's so-called Continental System.