Darn, just got this written and it disappeared, so will try again. Generally, speaking only of the cartridges and not the handguns shooting them, it was the Americans who were out of step with most militaries in choosing the .45 a.c.p. round. From the earliest development of military handguns, various 7 mm and 8mm cartridges were the most common. The first successful automatic pistol was the Borchrdt, in caliber 7.63 mm. (this cartridge is very similar to today's 7.63mm Mauser and the Russian 7.62 Tokarev cartridge). The 1896 Mauser ("Broomhandle") used a 7.63 mm. Browning developed the .32 A.C.P. (7.65 mm browning) for his small pistols, and this cartridge has been used my most militaries throughout the world, though usually as a substitute standard or personal purchase arm. The Germans experimented with a .30 Luger round, and finally adopted their famous 9mm round, which is still in use today. The Americans and the British had decided after their colonial experiences that smaller rounds were not suitable nor reliable for "putting down" an enemy, and the Americans brought back their frontier-era .45 LC round following the Philippine war, when the .38s they had been using were found to be inadequate. The Brits adopted the .455 for much the same reason. When the Americans began development of what became the Colt M1911, they essentially looked for a new round with most of the same characteristics as the old .45 LC, and thus developed the .45 a.c.p. Most countries never really considered the handgun as a significant combat arm, and tended to use it as a badge of office or in limited numbers for troops who could not be armed with a Rifle. Thus, there never was much interest in development of more effective rounds, which seems to explain the literally millions of guns chambered in .32 a.c.p. during WW2. During WW2, according to an American study, the actual casualties inflicted upon the enemy by pistols were very few-- thus explaining the lack of interest in developing new rounds. The British during ww2 used a lot of various types of .38 calibers and many 9mms. The 9mms were chosen apparently because the ammo was interchangeable with the 9mm used in submachine guns. The Russians became devotees of the submachine gun (again, smaller calibers), and did not pay much attention to pistols-- they realized that SMGs were easier and cheaper to make than pistols, so they had no incentive to develop new pistol rounds. Japanese use of pistols was primarily by officers as a badge of office, and their pistols were not in widespread use at any time-- thus, again, there was no real impetus to move beyond the state of the art of the inter-war period. The US, taking on much of the allied thinking (and with the pushing of NATO) officially adopted the 9mm in the 1970s (?) and continues to use it today. However, it must be noted that those forces charged with close combat with an enemy (notably the special forces and the Marines) continue to use the .45. Bottom line: no real advantages to the smaller rounds used by the Japanese, but no real reason to continue to improve their performance either, due to limited use. Most people with significant experience with handguns in combat will state that the .45 is more effective than any of the smaller calibers, but that unless your tactics are to close with enemy forces "up close and personal", it probably doesn't make a lot of difference.