Typically, pokemon used in war are large and powerful beasts. Although grass pokemon have a long history of military usage, for techniques like stun spore can be devastating with the wind at one's back, typically the grass pokemon used were those like Venusaur, sturdy beasts who could take a hit.
In the early 20th century, an innovative young general came up with the idea of using Vileplume gas to break the stalemate in the trenches of the Great Pokemon War. When its sleep powder and stun spore are combined with gunpowder and its allergy-causing pollen, a devastating poison gas was created which burned off the faces of the opposition and turned the tide in the war.
That general would go on to become a great conqueror, winning election from the prestige of his invention. He was bequeathed among his territory ownership of the gym in Celadon City, which is in the hands of his great-granddaughter Erika to this day.
The weapon was reverse-engineered after a few years of dramatic opposition by the nations of the world. The next decade saw many wars, but also many soldiers returning with the unbearable pain and injuries of Vileplume Gas: some said death was kinder. It was to ban this substance that the first international laws of war were developed, although they have since been vastly expanded beyond the original ban on Vileplume.