One prominent theme in The Face of Battle is historical contingency; that is, that the human arrangements, expectations, and outcomes of war cannot be understood without looking at a larger context. Despite the work of many war historians, Keegan exhorts his audience not to spend too much time absorbed in the immediate dramas of the battlefield. He advocates instead for a holistic history of battle that takes into account each event's surrounding geography, political conditions, technologies, and conceptions of the soldier and his functional purpose.

Another theme is the constant coevolution of rivals' war tactics and strategies. Keegan touches on different strategies throughout time, from the use of cavalry to conduct running charges in the Middle Ages to the strategies of troop spacing in the artillery trenches of World War I. Keegan does not think of war as a chaotic and unpredictable brawl, but as a system in which military leaders are constantly taking in and adapting to feedback from the front.