This article integrates the history of military theory – and the practical history of military campaigns and battles – within the broader history of knowledge. Challenging ideas that the new natural philosophy of the seventeenth and eighteenth centuries (the so-called Scientific Revolution) fostered attempts to make warfare mathematically calculated, it builds on work showing that seventeenth- and eighteenth-century natural philosophy was itself much more subjective than previously thought. It uses the figure of King Frederick II of Prussia (reigned 1740–1786) to link theoretical with practical military knowledge, placing the military treatises read and written by the king alongside the practical example of the Prussian army’s campaign against the Russians in summer 1758 at the height of the Seven Years War (1756–1763), which culminated in the battle of Zorndorf. This article shows that both the theory and practice of war – like other branches of knowledge in the long eighteenth century – were fundamentally shaped by the contemporary search for intellectual order. The inability to achieve this in practice led to a reliance on subjective judgment and individual, local knowledge. Whereas historians have noted attempts in the eighteenth century to calculate probabilities mathematically, this article shows that war continued to be conceived as the domain of fortune, subject to incalculable chance. Answering Steven Shapin’s call to define concretely “the subjective element in knowledge-making,” the examples of Frederick and his subordinate, Lieutenant General Count Christoph zu Dohna, reveal sharply different contemporary ideas about how to respond to uncertainty in war. Whereas Dohna sought to be ready for chance events and react to them, Frederick actively embraced uncertainty and risk-taking, making chance both a rhetorical argument and a positive choice guiding strategy and tactics.