During the Thirty Years' War, something like this happened to Gustav II of Sweden.
At the beginning of the war, John George I, Elector of Saxony, supported the Habsburg dynasty and Catholicism, and his own electorate was barely touched by the war. However, after Sweden entered the conflict, he allied himself with Sweden and supported Protestantism, and so the flames of war reached Saxony.
At the time, fewer people were killed by the enemy than died of disease. Even after corpses were buried, the fluids from their decomposition polluted the groundwater, contaminating the wells as well.
A typical village in the European Middle Ages often had a communal well. One could imagine what would happen when everyone in the village drank from such a well.
Brewing beer, meanwhile, placed great importance on water sources. Even the slightest impurity would give beer a distinct, easily detectable off-flavor. Moreover, brewing used very clean vessels, so compared with untreated water, beer was a harmless drink.
Strategists of that era planned beer reserves when arranging military campaigns. After soldiers arrived in an area, they would only let their horses drink water; they themselves would go to local breweries and warehouses to drink beer and quench their thirst. If there was none, they would search farmhouses and buildings, and only drink water after all the beer had been consumed.
Among the many misfortunes brought by war, merely having one's possessions taken away was considered fortunate—especially when the streets were filled with drunken soldiers.
Yet when Gustav's Swedish army crossed Krosty in Saxony, the local residents encountered a surprising force. Northern Europeans were generally tall and powerfully built, but more importantly, they observed discipline. They neither looted nor plundered, and when begging residents for food, they paid with clinking gold coins.
There was still unharvested grain