So, a thought hit me today. Japan, during WW2, lacked any sort of decent anti-tank weapon. So I began to think, what could they produce cheaply and still be effective? And then my mind came to the most simple, and arguably best, of anti-tank weapons. The panzerfaust. Cheap, easy enough for a child to both make and use, it's an amazing weapon.
So, let's assume Germany either sells them production rights or they just flat-out steal it. What impact do we have?
Imperial Japan during WWII, for the most part, never needed anti-tank weapons.
In North China there was so few enemy armoured vehicles it made no sense whatsoever to carry anti-tank weapons. These for the time across all the powers were typically fairly heavy to man-haul, bulky, and hardly suited to the thick hilly terrain of China and Manchuria.
Furthermore an 'RPG style' weapon would have not done the Japanese great wonders when light armour was sometimes deployed to take out strongpoints, and the Japanese were fairly well skilled with forms of satchel charges and bombs/grenades for other forms of duel purpose roles.
In the Pacific Theatre for much of the war the numbers of allied armour on station were far too small again for the Imperial Japanese forces to seriously have to consider anti-tank weapons. Indeed, before the island hopping campaigns by the Americans such a 'Japanese Panzerfaust' might have been actually harmful in diverting away explosives, funds and steel from light logistics craft and general supplies/ammunition which was often wanted for.
Had the Imperial Japanese army wasted time, money and resources on developing, putting an early form of RPG into service, then it would have likely seen very little use in many of the conflicts for much of the war and might actually have been a burden to infantry units, and strategic industry.
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EDIT: The Pacific Theatre was never Europe, asking questions of why did Japan/China not have anti-tank weapons, why didn't they invest more in tanks, heavy artillery, man-portable machineguns etc. is asking the totally wrong questions. Imperial Japan and China were two Great War era armies fighting in the WWII era, and
because of that never needed to invest/drastically upgrade their armour/anti-armour capacities. Not only this the terrain of north china was never really suited to armoured units, or heavy logistics. In the era roads were poor and little more than dirt tracks, railway lines were incredibly strategic and heavy equipment could only really follow these lines of communication. Beyond that, much had to be man-hauled by the infantry themselves.
This is partly why Imperial Japan stuck to the light infantry model for so long....it simply was the only way to fight in this kind of terrain, against this kind of enemy. Throwing anti-tank guns, or heavy artillery was pointless when their opponents didn't have armour or guns to bombard. So why waste resources designing equipment that isn't needed? This point is relevant in 9 out of 10 threads on Imperial Japanese, or Chinese equipment/doctrine of the era.