The M9 is the US military’s Beretta Model 92 service pistol. The M9 replaced the M1911 when it entered service in 1990. I remember when they did this. There was some concerns over stopping power when using the 9mm ball ammo. A few arguments in favor of the switch from 45ACP to 9mm was increased magazine capacity, the military was getting more smaller men and women in larger numbers and the 9mm has less recoil and standardizing the caliber with NATO. Never mind the USAF issued me a 38 Special, but that’s another gripe.
On 2005 the US Special Operations Command conducted the M9 replacement plan called the Joint Combat Pistol program. Some requirements were 45 caliber, striker fired, high capacity. Several manufactures submitted designs including S&W, Sig, H&K, Springfield Armory, Taurus, Ruger, Beretta, and FN.
On January 19, 2017 the Army announced it was replacing the M9 with the XM17, a military version of the Sig Sauer P320. This is a modular handgun system. Grips can be changed for better ergonomics, barrel lengths and calibers can be changed. The core is a serialized frame. It is a polymer striker fired pistol with a Picatinny rail.
The Army has chosen the 9mm over the 40 caliber. I suspect that they will move away from ball ammo towards newer 9mm that offers better ballistics and expansion. This is a result of the Modular Handgun System (MHS) study co authored with the Air Force. The Marines have expressed a desire to replace their sidearms by 2020. I wouldn’t be surprised if they didn’t choose this by then. The Navy SEALS have always liked the 9mm cartridge and until recently used the Sig Sauer P226. It wouldn’t be that much of a leap for the Navy to get on board. The SEALS are now using the Glock 19 now BTW. But during the MHS trials the Glock lost out because it wasn’t a modular system.