For the most part Trevor-Roper relied on investigations and interviews by hundreds of British, American and Canadian intelligence officers. He did not have access to Soviet materials. Working rapidly, Trevor-Roper drafted his report, which served as the basis for his most famous book, "The Last Days of Hitler" in which he described the last ten days of Hitler's life, and the fates of some of the higher-ranking members of the inner circle as well of key lesser figures. Trevor-Roper transformed the evidence into a literary work, with sardonic humour and drama, and was much influenced by the prose styles of two of his favourite historians, Edward Gibbon and Lord Macaulay. The book was cleared by British officials in 1946 for publication as soon as the war crimes trials ended. It was published in English in 1947; six English editions and many foreign language editions followed. According to American journalist Ron Rosenbaum, Trevor-Roper received a letter from Lisbon written in Hebrew stating that the Stern Gang would assassinate him for "The Last Days of Hitler", which they considered portrayed Hitler as a "demoniacal" figure but let ordinary Germans who followed Hitler off the hook, and for this he deserved to die. Rosenbaum reports that Trevor-Roper told him this was the most extreme response he had ever received for one of his books. In June 1950, Trevor-Roper attended a conference in Berlin of anti-Communist intellectuals along with Sidney Hook, Melvin J. Lasky, Ignazio Silone, Arthur Koestler, Raymond Aron and Franz Borkenau that resulted in the founding of the Congress for Cultural Freedom and its magazine "Encounter". In the 1950s and 1960s, he was a frequent contributor to "Encounter", but had reservations about what he regarded as the over-didactic tone of some of its contributors, particularly Koestler and Borkenau. Trevor-Roper was famous for his lucid and acerbic writing style. In reviews and essays he could be pitilessly sarcastic, and devastating in his mockery. In attacking Arnold J. Toynbee's "A Study of History", for instance, Trevor-Roper accused Toynbee of regarding himself as a Messiah complete with "the youthful Temptations; the missionary Journeys; the Miracles; the Revelations; the Agony". For Trevor-Roper, the major themes of early modern Europe were its intellectual vitality, and the quarrels between Protestant and Catholic states, the latter being outpaced by the former, economically and constitutionally.