By 1900, Berlin mechanical engineer, Richard Fiedler, was studying different nozzle design for spraying liquid, when he realize that gasoline could ignite from a pressurized tank that could catch on fire and used as a weapon.
British army also experimented with flamethrower, but find how short the fire goes inefficient. They also started to make an improved 2 ton flamethrower that could send flame over 30 yards. When it was introduced on July 1916, a couple flamethrower were destroyed within weeks. These were suppose to put German soldier into terror, however, British army were unable to get the trenches back. Because of these failure, British army abandoned it.
In 1978, the U.S. Military decided to ban the used of flamethrowers. They are not currently used by American soldiers. The decision to ban flamethrowers was voluntary. At that time officials clearly expressed that they were not effective in modern battle.