The Thirty Years' War drove Fleming to Holstein, where in 1633 Frederick III, Duke of Holstein-Gottorp, engaged him as physician, courtier and steward. Towards the end of 1633 the Duke sent Fleming with Adam Olearius as a member of an embassy to Russia and the Persian Empire headed by Otto Brüggemann and Philipp Kruse. Fleming was outside Germany for almost six years, much of them in the two foreign empires. Travelling into Russia, Fleming was in an advance party of the embassy which went to Novgorod, where he remained while negotiations went on with the Swedes and the Russians. At the end of July 1634 the ambassadors joined the party, and the embassy proceeded to Moscow, arriving on 14 August. After four months in the capital city, the Holstein embassy departed again for the Baltic on Christmas Eve, 1634, and on 10 January arrived at Reval (now Tallinn) in Swedish Estonia. While the ambassadors continued to Gottorp some of the party, including Fleming, remained in Reval. In the event, Fleming was there for about a year, during which he organized a poetry circle called "the Shepherds". Not long after his arrival in Reval, Fleming began his courtship of Elsabe Niehus, the daughter of Heinrich Niehus, a merchant originally from Hamburg. He wrote love poems for her, and they became engaged to be married. In 1636 the embassy proceeded to Persia, by way of a further visit to Moscow, and Elsabe was left behind. Fleming's "Epistolae ex Persia" were four letters in verse written during his time in Persia, between 1636 and 1638. The embassy was at Isfahan in 1637. On returning to Reval, Fleming found that Elsabe had married another man and became engaged to her sister, Anna Niehus.