Water is one of the most precious goods for sustaining life on the planet. The ancient peoples knew this truth and many times they tried to deprive their opponents of this precious good, or even worse, to use it as a weapon against their opponents.
The rationale was simple. In any case, either the troops of the attacked, or the entire population of a besieged city, for example, needed water. If this water became undrinkable then in both cases a serious problem would arise. There were many ways of poisoning the water. The most common were poisonous plants and mushrooms. Another way was to throw animal carcasses into the wells.
This second method was unsuitable for poisoning running water, but was extremely effective in wells, cisterns, etc. They used poisonous plants and often boiled them and poured the resulting juice into the water. For the poison to work effectively, however, it had to be renewed at regular intervals.
Needless to say, the poisoned water was unfit even for watering the fields, as it also contaminated the soil. An unpleasant example is the Phocian city of Kirra, near today's Itea. Its inhabitants invaded in 590 BC. estates of the oracle of Delphi. Immediately the Amphictyonic conference declared a holy war against the sacrileges (First Holy War).
Athens and Sicyon were placed at the head of the alliance. The allies were sworn by the Delphic priesthood to wipe the city from the face of the earth, to destroy even the fields and fruit trees. The priests of Delphi said that Apollo had cursed the sacrilegious city. Her soil would no longer bear fruit and the women's children would be born deformed.
So just as the god would show no mercy, so the allied troops would have to be merciless. And indeed it was. The allies, under Cleisthenes and Eurylochus, besieged Cyrra. In the absence of siege engines, but also because of the mountainous terrain, the siege was not progressing satisfactorily for the allies. But then they discovered a hidden pipeline, which carried water into the city, from a neighboring spring. They immediately broke the pipe, cutting off the water supply.
After a few days, however, during which the besieged suffered from thirst, the aqueduct was repaired and water began to flow again into the city. Unfortunately for the besieged it was poisoned with black weed. The inhabitants all fell ill and the city was taken without further resistance. Apparently after their victory the allied troops watered Kirra's fields with poisoned water. Trees and plants dried up and not only that.
Even when the traveler Pausanias visited the site 740 years later the soil was still poisoned. "The plain around Kirra is uncultivated because the land is still cursed and the inhabitants cannot plant trees," writes Pausanias, who attributes the plan to poison the waters to the wise Athenian legislator Solon.
This form of total war was used again against the Persians of Xerxes. Leaving their city at the mercy of the invaders, the fugitive Athenians did not fail to poison their springs, wells and cisterns. Later, during the Peloponnesian war, the Athenians accused the Spartans that for the great famine that hit Athens in 430 BC. was responsible for the poisons that had been thrown into the water.
The charge was unfounded. Some researchers believe that the Syracusans had also poisoned the waters of the rivers and springs during the Sicilian Campaign of the Athenians. However, this view is not documented by any ancient source.
On the contrary, the Syracusans seem to have poisoned some water sources around their city, during the great Carthaginian attack in 397 BC. In the middle of the 4th century BC Aeneas the Tactician spoke of the use of poisons in water in siege operations.