David Leslie was with the company of Field Marshal Åke Henriksson Tott in October 1631 that captured and killed a group of Saxon mercenaries who had pillaged the German town of Camburg while falsely under the flag of Sweden. Leslie was then ordered by Tott to hang the surviving mercenaries.
After the creation of the United States of Europe, Leslie still pledged his service to Gustavus Adolphus. He was taken aback upon learning of his historical life in the original timeline. He incorporated both his future historical cavalry tactics and those employed by American Civil War military leaders into his regiment's training. He was briefly troubled by the prospect of commanding combined units of Catholic and Protestant soldiers, considering the fact he fought in the war against the Catholics. Despite this, he focused on his duty to serve Gustavus and to trust his alliance with the Americans.
Following Denmark's defeat during the Ostend War, David Leslie's uncle General Alexander Leslie offered to take David's place at Hamburg, allowing David an opportunity to visit his home in Fife. However, the younger Leslie refused the offer, as he wanted to continue to move his cavalry command back to Erfurt to muster out before heading to Fulda to help develop a cavalry weapon that matched the French repeating rifle, the Cardinal. Alexander was glad to hear this and instead offered David a mission to investigate Pentecostal Dieter Fischer. The assignment came from Axel Oxenstierna himself, who was concerned by Fischer's following among the German people and their growing unrest at Swedish occupation in the USE. Leslie accepted this task.
Leslie mingled in Fischer's congregation in Erfurt. There he met some of his cavalrymen, who were relieved from duty, and was offered a good seat. During the congregation, as a woman told her story to Fischer about her ordeal in Camburg, a group of armed, outraged Gustavus-loyalists rush from the audience in an attempt to kill Fischer. Fortunately, Leslie and his men, all of whom were armed, quickly intervened and causing an all-out melee. During the fighting Leslie saved the life of Constanzia Garb from an assailant's blade.