A couple years ago, URR had a nice post on the Marine Corps shift to the 120mm rifled mortar system known as the Expeditionary Fire Support System. Basically, it is a rifled 120mm mortar, its associated ITV prime mover, and the associated ammunition caisson and its prime mover, another ITV. General Dynamics, the prime contractor for the system, put together a nice little marketing video. Lots of shooty, even some splodey.
There’s a major, major difference between the Marine EFSS and the Army’s own 120mm smoothbore mortar systems. And it’s not really so much the guns themselves. It’s the organization of fire support assets.
A Marine Division has three infantry regiments, and an artillery regiment. The Marines have elected to replace their light 105mm howitzers in the artillery regiment with the EFSS. That means the division’s artillery will lose significant range, but will also gain a much greater ability to land early via vertical envelopment using the MV-22 Osprey, and that the small size and light weight of the EFSS will allow battalions and batteries of fire support to move quickly right behind the supported infantry regiments and battalions. It is a fairly bold shift, but the Marines probably know better than I what their fire support requirements are. One other major impetus for shifting to EFSS is that space on amphibious shipping for artillery is incredibly tight. EFSS has a very small footprint, which makes finding space for it much easier. Or rather, not taking up as much space as a conventional 105mm artillery battery frees up space for other vehicles and equipment the Marines really want to bring along, but previously had no footprint for.
The Army, by contrast, doesn’t have the same shipping and footprint constraints. Further, the Marines have, historically, only operated in division or larger sized formations since World War II. The Army, by contrast, has always had (at least theoretically) the ability to field corps and field armies. And each of those formations had their own artillery to reinforce the fires of divisional artillery. For instance, today, each Brigade Combat Team has its own Field Artillery Battalion, to support its maneuver battalions. The division headquarters controlling the BCT might well have a Fires Brigade attached to effectively double the artillery available. In the Marines, there simply isn’t any artillery above the division level.
In the Army, 120mm mortars belong to the infantry and combined arms* battalion commander, in the form of a mortar platoon organic to each battalion. That is, they are not an artillery weapon, but an infantry weapon, one of many supporting weapons organic to the maneuver unit.
Both the Army and the Marines have smaller mortars, 60mm and 81mm, that are infantry weapons, belonging to the rifle company or the infantry battalion, though how they are distributed differs in detail, if not in effect.
*Combined Arms Battalions are the maneuver battalions of Armored Brigade Combat Teams, and consist of a battalion with two tank companies, and two Bradley mechanized infantry companies.