The basic idea of a flamethrower is to spread fire by launching burning fuel. The earliest flamethrowers date as far back as the 5th century B.C.
Flamethrowers were not widely used in World War I. While flamethrowers were developed and utilized by several countries including Germany, they were not a major weapon in the war and were not used on a widespread scale. Instead, they were primarily employed in trench warfare to flush out enemy soldiers from dug-in positions. The use of flamethrowers raised ethical concerns and was considered inhumane, leading to their limited deployment.
Take a look at these 4 rare images of flamethrowers used during World War One.