Flamethrowers are an incendiary device that project a long stream of fire by igniting a flammable gas or gas and projecting it over a long distance. Modern flamethrowers use propane or natural gas because it is considered safer. Richared Fiedler is credited with inventing the modern flamethrower, which uses 3 cylinders used for holding the flammable gas, the projectile gas, and the igniter in 1901, but it wasn't used in battle by the Germans until July 30th, 1915 against British forces at Hooge.
Modern flamethrowers were first used during the trench warfare conditions of World War I by Germany; their use greatly increased in World War II when the Allies began using it. They can be vehicle mounted, such as a tank, or man-portable.
The flamethrower was a very potent weapon on the battlefield, especially on unprepared soldiers that weren't psycologically prepared for such a weapon. With a distance of up to 80 meters, this made the weapon very effective against trenches, bunkers, and battlefield fortifacations. With this, it leveled the playing field when soldiers were faced with bunkers that normal weapons couldn't pennetrate.
There have been several petitions to ban this weapon due to the horrific death it causes. When a flamethrower shoots into an enclosed space, such as a bunker, it will consume the entire room in flames, because it will have nowhere to go. When soldiers would scream from the pain of being burned alive, the fire and smoke would engulf their lungs, essentially cooking you from the inside out. If you were lucky enough to live through the initial burst, there would be soldiers waiting for you to come out and they would pluck soldiers off one by one as they exited. In turn, it was also very dangerous to be operating a flamer. The bulky weapon made soldiers stand out on the battlefield, making them a target for snipers and infantry. However, the explosiveness of the fuel in the tanks is often exaggerated in movies. Flamethrowers have the fuel separated into three different cylinders in the pack, making it very unlikely to explode since the elements were separated. Normal ammunition would not start the fuel on fire either, it would take incendiary rounds to have a higher likelihood of igniting the fuel. When flamers were mounted on tanks or armored vehicles, the fuel tank would be heavily armored to prevent explosion. Even today, they are still mounted on tanks for close quarter battles. It is rare for them to be used as hand held today due to their weight.
When WWII came, the United states relied on the flamethrower heavily in battle. The Japanese were notorious for digging into the hills and creating a series of caves and tunnels, much like in the Vietnamese in the Vietnam War. The Japanese very rarely surrendered, and would fight to the death. Rather than sending soldiers into the tunnels, where the Japanese would be waiting for them, a soldiers would stand at the entrance and smoke and burn them out. This was a very common practice when the U.S. was island hopping and at Iwo Jima.