After the First World War, the flamethrower was quickly discarded, since most armies considered it to have been a specialized device for the peculiar conditions obtaining in the trench warfare in Flanders. It reappeared in Italian hands in the Abyssinian campaign of 1935, when a tank-mounted flamethrower was employed, and also in small numbers in the Spanish Civil War, in both man-pack and tank-mounted versions.

During the Second World War, the German army used flamethrowers in the Polish campaign and also in the advance into France and Belgium in 1940, particularly against the Belgian forts such as Eben Emael. After this, they saw little use since they did not fit in with the tactics of open war. The British Army, on the other hand, began development later in the war with the intention of using flame during assault operations against the coast of Europe.
 

Much British work was done in 1940 on developing devices; flame guns for Home Guard use, flame traps, and flamethrowers shooting flame into the sky to attack troop-carrying aircraft during their landing runs were all developed to varying degrees of efficiency. More serious work went into the development of tank-mounted flame throwers, the Crocodile and Wasp being the principal models, and were used to good effect in north-west Europe in 1944-45. A manpack model, known as the 'Ack Pack' was produced, though this was largely intended for operations in the Far East, particularly against Japanese pillboxes and bunkers where the defenders were willing to fight to the death.
 

The Ack Pack consisted of a ring-shaped container, similar to a lifebuoy, carrying 18 litres (4 Imp gal) of fuel; in the centre of the ring was a spherical gas container holding gas at 140.6 kg/sq cm (2000 lb/sq in) pressure. From the bottom of the fuel tank a hose ran to a nozzle unit which had two pistol grips and a firing trigger. Around the nozzle was a revolving cylinder with ten chambers, each holding a special ignition cartridge. On pressing the trigger, a valve released fuel under pressure, and an ignition cartridge was fired, igniting the fuel which then gave a jet to about 36 m (120 ft) range.

In 1940, the US Army began developing a manpack thrower, which was standardized in late 1941. The first combat use in American hands was on Guadalcanal in January 1943, when the 2nd Marine and 25th Infantry Divisions used them to deal with Japanese pillboxes. Later operations were to have varied results, some successful and others disastrous. This was due to equipment malfunctioning; much of which was caused by the rapid effects of heat and damp experienced in the Pacific. Difficulty arose over the fuel, and it was not until rigorous maintenance was insisted upon and a new fuel, based on napalm, was adopted, that flamethrower operations began to succeed.
 

Generally speaking, records tend to show that the flamethrower is a mixed blessing. To obtain results it demands a high state of specialized training, not only for the thrower operator but of his supporting squad, and this training is difficult to arrange when other, more basic, skills appear to be more vital. Since flame is only used occasionally, the tendency is for the trained team to be dispersed and used in other tasks when things are slack. When flame is suddenly needed, the trained squad is absent and an inexperienced team is hurriedly assembled. They usually fail, the failure is blamed on the flame device, which takes another drop of esteem. It was noticeable that where trained flame operators were in frequent use, as with the US Marines and infantry in the Pacific and the British 79th Armored Division in north-west Europe, flame gave excellent results and commanders were willing to employ it. Where commanders were not used to it, they tended to shy away; there were several instances in Europe in 1945 where it might have been used to good effect but where conventional weapons were used instead