This paper addresses the issue of how water played a role in ancient conflicts, from the poisoning of water sources to flooding, to stop the advance of enemy armies. It deals with military actions quoted by several ancient Greek and Roman authors, who in some cases narrate these experiences first-hand. Although many abhorred such actions, they were considered tactical expedients to resort to, as cited by the war manuals of the time. The analysis starts from the ‘manual’ Strategemata of Sextus Julius Frontinus, in addition to other references left by historians and chroniclers of different periods. It continues with the evaluation of the impact of the intentional actions of water contamination described by the ancient authors, according to present toxicological and health knowledge.