Bit of column A, bit of column B. Flame weapons have been around for a large part of human military history, but we first saw what would be the basis of the modern "man-portable" backpack flamethrower appear in the first world war with the Germans, where they immediately proved their worth: The German military had been developing flame weapons from the early 20th century, and had made limited use of them against the French in 1914, but the first proper use was at Hooge in Flanders. The effects were devastating, resulting in temporary loss of British front line trenches in the affected area, and the loss of 31 officers and 751 other ranks.

The back pack German flammenwerfer (literally flame thrower), powered by compressed gas tanks and shooting burning oil, would be the basis of other nations future weapons up to and into Second World War. However, the WW1 experience with flamethrowers would be similar through following conflicts as well. Although devastating, Allied troops quickly found counters to German flame weapons. Germany too noted that they were effective weapons, but also that Allied soldiers made it their mission to neutralize the weapons wherever they appeared: Several accounts I have read of British soldiers encountering German flame troops at the Battle of the Somme stated that all effort was made to kill them when they appeared: One even noted that one flame trooper was shot several times but did not die, and that he was apparently wearing some form of body armour, whereupon he burned several British infantryman to death before being killed with a shower of grenades.

This highlights the main problem with backpack flame throwers: The main reason that make them ever more effective than shotguns and grenades in clearing fortifications (extremely lethality from both burning and suffocation, but also enormous psychological effect) is also what limits them on the battlefield: Not only does man portability bring about severe drawbacks (heavy weight, limited fuel and poor effective range, often less than 30m), the main reason is that they are so psychologically devastating, any flamethrower operator on the battlefield immediately becomes the biggest target for enemy fire. This meant flamethrower operators on all sides in both conflicts had among the lowest life expectancy of any soldier. Although stories your here about flamethrowers exploding when shot are almost totally untrue: Being pressurized with gas such as nitrogen, the contents are unlikely to burn when a thrower is shot. In fact, one of the main problems of flamethrower design was lighting the fuel, as thickened oil and the napalm use later is surprisingly difficult to ignite. However accidents did occur, such as at Fort Douaumont during the Battle of Verdun, where nearly 700 German soldiers perished when flamethrower fuel was ignited by a cooking fire in an ammunition gallery.

There were several ways round this: German tactics in late WW1 integrated flame weapons into stormtrooper assault squads, using shock and force of action to devastate weak-points in enemy trench lines and bypass fortifications. In WW2, effort was made to disguise flame troopers. One example is the Soviet ROKS-2 flamethrower, designed to roughly resemble a standard Mosin rifle and have its fuel tank hidden inside a normal knapsack.

Generally though, the limitations of backpack throwers had one easy solution: Scale it up, and mount it on a tank. Not only did this improve survivability, but effectiveness increased hugely: tanks could carry gallons of fuel and plenty of compressed gas, allowing longer firing times and far greater range, often enough to shooting flame over ranges up to 100m. Many nations used flame tanks in WW2, but among the most notable and effective were designs such as the British Crocodile, and the American M4A3R3 "Zippo", both of which were extremely effective against enemy structures.