Outside of the seldom used Thompson and M3A1, I've noticed that the United States army doesn't really utilize submachine guns, and when it does, it uses foreign models. Can you explain this American aversion to the SMG?

The Submachinegun by and large has fallen by the wayside in terms of its general usefulness.

During the interwar and World War II Era the United States trialed and adopted a number of submachineguns from a number of manufacturers, including but not limited to the M3A1 "Grease Gun", M1 Tompson, Reising Model 50, and the Ud 42 (in small numbers). By and large these Submachine guns proved acquitted for the intended role of the US Armed Forces Submachine guns: To provide Infantry NCOs and Low-Rank officers, Vehicle Crews and other personnel a compact automatic weapon capable of providing relatively high volumes of fire at distances less than 100 meters compared to the then standard infantry rifles.

Following the war, the M1 and M3 remained largely in service in similar roles, with a number being replaced by the far more flexible M1 and M2 carbines, which fired a 7.62x33mm Cartridge at a muzzle velocity of 606 meters per second from the semi or fully (M2) automatic rifle. These rifles had a longer effective range of about 200-300 meters and were generally more accurate than their SMG counterparts at range. The M1 Carbine may not have had as fast of a fire rate as the Thompson, nor as big a magazine, but the M2 carbine fired at 850 rounds per minute from a 20 or 30 round magazine and was fairly moderate of recoil, while also weighing a mere 2.6 kilograms loaded.

During the late 1950s and early 1960s Submachineguns further fell into more specialized roles, like those for Military Police, Special Forces, or vehicle crews largely due to the United States adoption of the M14 series of rifles, which were semi-automatic feeding from a 20 round detachable box magazine. To complete homogony throughout the squad and platoon, most riflemen were armed with these rifles so they could simply share ammunition where possible. The M1 largely was phased out by this point in most of its previous roles, but saw some limited use in the early days of the Vietnamese War.

Notably, the US DID actually adopt a new submachinegun in this era, one considered by some to be the best developed: The Swedish Carl Gustav Kulsprutepistol m/45, which was very popular with US Special Forces operating in the area. So popular, in fact, when Sweden placed an Embargo on the United States as a result of actions in Vietnam, the US Contracted Smith and Wesson to build an unlicensed copy, known as the Smith and Wesson Model 76 Submachinegun.

The adoption of the Stoner/Armalight XM16 and M16 rifles slowly began the demise of the Submachinegun. They replaced even more roles, but still they were in use with Special Forces and Armored Vehicle Crews.

Through the 1970s and 80s the US worked more and more on the development of the M16 series 5.56x45mm Small Caliber, High Velocity ammunition to increase its performance from a variety of weapons in a number of ways. Major contributions came from the Belgians and the British, of course, but this is a dramatically different topic. As gunpowder primers, bullets and barrel twist rats improved, the M16 series slowly became more and more effective at using shorter barrels, something that most rifles were unable to take away from Submachineguns. The previous M14 rifle in 7.62x51mm NATO was optimized for barrels of generally longer lengths than the M1 Thompson, m/45 and M3A1 Grease Gun, and the M16s 5.56x45mm ammunition was still optimized for a barrel of about 20 inches, while al these had barrels of less than 250mm, while the M16 and M16 both had barrels of over 400mm. This was basically the major pro of most Submachineguns of the era; compactness.

The H&K MP5 busted onto the scene in the 1966 but was not really popularized until the late 1970s and early 1980s, as it was seen more and more in the hands of Special Forces. The MP-5 is an excellent Submachinegun. Chambered in 9x19mm NATO, it comes in a number of versions and was considered to be the SMG to have in the 80s, mostly due to its generally all around excellent performance. The ability to place a silencer on the weapon and have it be effective as it was was, too, a great pro. Rifle round require special tooling to work well with suppressors due to the supersonic crack emitted by the bullet, in addition to a number of other issues associated with fitting sound suppressors to rifles like the M16 (Again another discussion to have), but the MP5s 9x19mm cartridge did not. While supersonic in standard Full Metal Jacket loads, a special heavy weight subsonic load existed, allowing the MP-5 to be suppressed very efficiently, which popularized its use with special forces operators, as well as its legendary rigidness and performance.

As time went on the SMG slowly lost favor to the Assault Rifle in its last real bastions of Law Enforcement and Special Forces. As the M16 and its European counterparts, like the FAMAS, FN FNC, L85 and various other rifles took shape and development, it seemed NATO had effectively created the "ideal" multipurpose cartridge for all general users. The low recoil of 5.56, combined with its effective fire range of 400-600 meters and low weight made it a superior choice to the comparatively heavy pistol rounds and their heavy guns that fired them at less range. The shortening of barrels over time gradually allowed the SMG to be largely replaced by the rifle, mostly the M16 and M4 carbine in the US. The last two bastions of the SMG are armored vehicles and law enforcement. The low penetration of pistol caliber projectiles like .45ACP and 9x19mm NATO, compared to 5.56x45mm NATO, ensure that law enforcement doesn't risk over penetration of a hostile in instances in which they need to use lethal force and, therefore, place civilian lives at risk. M3 Submacineguns were seen as recently as 2003 in the US Armys inventory in the Middle East, serving faithfully as a crew weapon in tanks and other armored vehicles as personal protection, though its probable that none are still out there. Special Forces, being as flexible as they are, still use them for a number of missions, but not nearly as much as they once did.

The US has little reason to adopt or really develop a new Submachinegun in the modern era, as the MP-5 is effective enough as it is, and most roles it would have filled in 1944 are today filled by the M16A2, A4 or M4(A1) rifle. They had little reason to do so as more effective and efficient small arms projects were already underway to replace what was seen, in many circles, as a weapon rapidly approving its obsolescence. Submachineguns today still have a niche, and will for a few years, but are now generally only found in this niche and, slowly bit surely, will be replaced.