Apart from the United States, it's been a 9mm world for a hundred years. In the US, it's been dominant since about 1980. The S&W Model 59 (fourteen round magazine 9mm pistol) became wildly popular with US police departments in the 1970s, along with (shortly after the 59's introduction) the Beretta 92. The Browning Hi Power (introduced in 1935) already had a faithful US following when these two came along, but 9mm didn't really catch fire until the early to middle 1970s (thanks to the S&W Model 59 and Beretta 92). By the 1980s, 9mm was dominant here.
