Leslie was made an imperial chamberlain by Ferdinand II in 1634. His military rank was increased to a lieutenant field marshal and head of the bodyguard for the King of Hungary, and he was given command of two regiments. Just after his promotions, in the summer of 1634 he led a successful siege on Regensburg. In September 1634 he fought at the imperial victory at the Battle of Nordlingen. In the months leading up to the Peace of Prague, Leslie was in Rathenow in Brandenburg. Historian David Worthington claims that of all the assassins of Wallenstein, Leslie was rewarded the most and ultimately his career grew the most out of that event.