Even in the early days of humanity, people were lazy. They wanted a way to kill each other without having to go to all the trouble of bashing each other's heads in with rocks. So they turned to better living (briefly) through chemistry. Find out how wolf crap and chilis won wars.
No one knows the first time a human came home from a long day of sticking pointy things into other humans to say, "Man, there has got to be a easier way." But we do know that the first time was not the only time. The ancient world is rife with chemical warfare. Chemical weapons have been used for millennia to burn, blind, suffocate, poison, or just plain irritate the enemy into submission.
Any substantive discussion of ancient warfare has to include the good old Greeks. They managed to come up with the idea of using flame throwers in combat to burn people alive. Greek Fire was a famed weapon, although its exact composition is still not understood and some say its abilities were exaggerated. It started with those lovable Spartans building up a blaze, setting a pipe up behind it, and using a bellows to blow air down the pipe, blowing flames toward the opposing side. Once the concept proved effective, Byzantium got chemicals involved; chemicals probably derived from the petroleum that came from around the Black Sea. They used chemicals to make fire portable. Instead of huge heaps of coals, they could direct a stream of flame. When they applied this to naval warfare, they were a devastating force. Not only did the flaming chemicals burn enemy ships - they floated on water and burned any survivors of sinking vessels.
Ancient China is just as unavoidable as Greece. The Chinese took the concept of the flamethrower and went wide. They improved on the Greek's flamethrowers by adding a second bellows, so the fire was continuous. They put flamethrowers on wheels. They even found a way to hand carry flamethrowers onto the battlefield. They filled a shaft of bamboo with flamable chemicals and attached it to the end of a long spear. When the chemicals were lit, they shot fire several feet out from the lance, and kept going for five minutes. To put it in perspective, many modern flamethrowers don't keep burning for five minutes.
Suffocation and Irritation
Chinese soldiers also used gases of all kinds. When the enemy tried to tunnel under walls, they blew smoke from furnaces into those tunnels, suffocating them. When the enemy tried to attack a city, they fired smoke bombs that caused bleeding from the nose and mouth. They even devised formulas that used wolf and sometimes human feces ground into powder and mixed with arsenic. This was said go right through protective clothing and cause blisters. It was used much the way nonlethal gas is used today - as a form of riot control.
Down in Brazil they also used smoke to control crowds. This smoke didn't blister the skin, but it drew tears from the eyes. Ground up chili peppers were burnt over fires and the smoke from the peppers was fanned towards the enemy. Many people reading this will have been careless in the kitchen and touched their eyes after handling spicy foods. Most of those people will only have done that once. The smoke, loaded with capsaicin from the peppers, blinded people with tears.
Poison, Poison, Poison
Why should fighters make their enemies tear up, though, when they could make them keel over instead? Curare is a generic name for poisons used in South America. Most often seen doing the impossible in pulp paperbacks, it does have its uses in reality, too. Most types of this poison were made with the bark of the subtly-named Strychnos toxifera. The more discerning poisoners added a little snake or ant venom, threw the ingredients in a pot of water, and boiled them for a couple of days until the mixture became a paste. To make sure the recipe was right, brewers would count how many hops a frog could take after being pricked with an knife dipped in the stuff. Curare kills by slow, creeping paralysis that leaves a victim conscious until it attacks the muscles that control breathing. Curare was most often used in hunting - since it only poisons through contact with blood and can be ingested in animal meat without harm to humans.
Poison, of course, was not just the weapon of hunters and armies, but the weapon of Emperors as well. Romans used it, although it was frowned upon, to poison the drinking water of Germanic tribes when they were being belligerent. What was good for the goose was good for the gander - Romans used it to kill each other when they were being belligerent as well. In fact, a woman named Locusta was said to be a universally known professional poisoner. And a professional poison homicide investigator, hired by the state. (At last a way to make a new Law & Order spin-off that's interesting.)
Poison was such a versatile addition to weapon that it was used almost everywhere. Ancient Iranian archers would infect their enemies by dipping their arrows in manure and blood. The Romans used poison in water supplies again in Asia Minor when faced with guerilla warfare after having destroyed the region's army. Hawaiians and South Americans dipped their arrows in secretions from the skin of many different kinds of poison dart frogs. Some of these frogs are so poisonous that they can kill or severely injure people simply by being handled too long. It's a shame that more research hasn't been done on the uses of the poison dart frog, because it could have made for one of the most embarrassing and funniest ways ever to win a war through chemical weapons. Instead of shooting and stabbing and gasing and bludgeoning, one side could have gone up to the other and said, "Here. Hold this frog. We'll be back in minute."