In Sartor Resartus, which he wrote that year, Diogenes Teufelsdröckh's father is a retired sergeant in Frederick's army. Carlyle asked Karl August Varnhagen von Ense in 1840, "Did anyone ever write an adequate Life of your Frederick the Great?"[4] He began reading a new biography of Frederick by Johann Preuss in 1845, shortly after he had completed the manuscript of Oliver Cromwell's Letters and Speeches.[5] The biography prompted him to declare, "Certainly if there is a Hero for an Epic in these Ages—and why should there not in these ages as well as others,—then this is he!"[6] He began research for his work in 1851, after the completion of Latter-Day Pamphlets and The Life of John Sterling, touring Germany in the autumn of 1852.