I don’t think there has ever been any question that fire can be used as a weapon. The main change throughout the millennia is the delivery system.
One modern delivery system is the Molotov cocktail. Cheap and easy to make using accessible ingredients, these little bombs are easy to carry and can do some big damage.
As demonstrated in many, many movies, the Molotov cocktail is a bottle filled with gasoline, using a gas soaked rag stuffed in the opening as a wick that is lit to start the fire. You throw the bottle and when it breaks, gasoline and fire spread everywhere. Easy peasy.
But Hollywood doesn’t always tell the whole story. Molotov cocktails generally have a few extra ingredients to increase their effectiveness. The most damaging Molotov cocktails add a little something extra to the gasoline or ethanol. If you add some tar or motor oil, you get a more sticky mixture. Sticky is good in this case. When the bottle breaks, you want the fire to stick where you’ve thrown it. These add-ins also provide extra thick smoke, which is good to disorient your enemies.
I would guess that people have used variations of Molotov cocktails since there was glass and accelerants. Tossing a lit kerosene lantern to purposely start a fire is, in my view, a precursor to the more standardized modern version. And yes, I’ve watched a lot of Westerns in my life.
It’s turns out we can thank the people of Finland for naming a weapon they standardized and used extensively against the Soviets. In 1939 the Soviet foreign minister, Vyacheslav Molotov (1890-1986), signed the Molotov-Ribbentrop Pact with Nazi Germany. Billed as a non-aggression pact between the two countries, it also secretly divided territory between them, giving the Soviet Union control of Finland. Again.
Then the Soviets invaded Finland and started dropping bombs. When the news reported on the bombs, Molotov declared that they were dropping food and supplies. The Finns started called the bombs “Molotov bread baskets”. It only made sense to call their weapon of choice the drinks portion of the meal, hence the Molotov cocktail.
These small weapons were extremely effective against tanks. If you have enough Molotov cocktails and they are sticky enough, you can keep a tank aflame and completely surrounded by thick, black smoke. And the Finns did have plenty of Molotov cocktails because, surprising, they mass-produced them in a distillery during the war. Way to go, Finland!