Military expenditure and the number of service personnel are the two features most commonly used to compare national military power. The question remains, however, to what extent these reflect the real-world situation. This study aims to provide an answer by using economic and military data about conflicts between great powers over the last 160 years. Correlations of War data are employed to show that the relationship between pre-war military expenditure and army size on the one hand and outcomes of war on the other, is blurry to say the least. States with higher military expenditure prevailed in only six of the nine conflicts between great powers examined in this research. Only four of the nine were won by the state with the larger peacetime army. Using the case of the Franco-Prussian War, this work illustrates that even the superiority of both mentioned variables cannot ward off a crushing defeat, let alone ensure victory. A nation’s military power stems from its ability to adapt effectively to the realities of modern warfare. This is what neither high military expenditure nor sheer soldier numbers can guarantee.