Ethologists have observed that some animals use organised violence against members of their own species. Some well-known scholars call this form of animal aggression war, deliberately omitting the quotation marks. And how was it in antiquity? Was war believed to be part of the animal world? The aim of the presented article is to analyse the ancient written sources in search of correlations between the notion of war and animals, and to determine what conceptual assumptions lie at the heart of this correlation. In light of the preserved records, it seems that the conviction about the existence of an inherent correlation between war and the animal world emerged slowly; that is, basic military terms (polemos, machē, etc.) were used with respect to animals reluctantly in the beginning and rather metaphorically, but gained popularity over time. Even though animals appear in the military context beginning from Homer's Iliad, Aristotle was most probably the first one to explicitly formulate a thesis about the presence of war (polemos) in the animal world. Aristotle's concept of war comes down to a series of duels fought between animals. It seems that what made it easier for Aristotle to transfer the notion of a battle (machē) from the human world to the animal world was the Greek tendency to present duels between soldiers as being equivalent to a clash between two armies.