On the issue of Light and Medium Ground Units especially, I am not entirely convinced but still rather doubtful that conventional Mountain, Marine, Airborne/Air Assault, and Motorized Divisions are necessary, or if so, only under even more limited circumstances than now. Many countries maintain substantial forces of Light and Medium Ground Units. Many of these units are intended to be more for power-projection than defence, even though they lack heavier armour and weapons as well as restricted supplies. Such units as these I suspect that they are rather more suited to the Second World War, in which many of them (especially Marine and Airborne Divisions) were formed, than now.
The US Army, for example, has 1 Airborne, 1 Air Assault, and 2 Light Infantry Divisions (basically even more lightly armed and equipped Airborne Divisions but not on jump status - one of which, the 10th Mountain Divisions, carries the Mountain title as a traditional, not actually operational, designation) in the Active Army, and 1 Light Infantry division in the Army National Guard. To complicate matters, many of the brigades of these specialized foot infantry divisions are in fact motorized using the LAV III Stryker armoured car. Until less than 20 years ago, the US Army included a Motorized Infantry division.
The US Marine Corps has 3 Active and 1 Reserve Marine Divisions (essentially the traditional Foot Infantry Divisions supported by medium artillery and small numbers of heavy tanks and IFVs as well as armoured cars (LAV-25 Piranha), but with specialized amphibious assault training and equipment).
The PLA now maintains 3 Airborne Divisions (1 still forming) and is plans to form 3 more, supported it seems by some Air Assault units. The PLA also maintains several Light Infantry Divisions as well as many Foot Infantry Divisions supported by medium artillery and large numbers of MBTs and possibly some limited APCs. The PLAN maintains 2 Marine Brigades, motorized infantry with artillery and tank support.
The Russian Army maintains a few Airborne Divisions (with some light armour)as well as a few Air Assault Brigades. The Russian Navy maintains at least 2, maybe more Naval Infantry Brigades, motorized infantry with artillery and tank support.
The British Army maintains (a somewhat imperfectly parachute-capable) Air Assault/Airborne Brigade, while the Royal Marines maintain a Commando Brigade (including Army troops) specializing in amphibious warfare. Army Motorized ("Mechanized") Brigades exist.
The French Army maintains small Airborne, Airmobile, Alpine, Motorized, and Marine Divisions (the size of brigades elsewhere), although the Navy maintains a battalion of Marine Commandos.
Many other countries maintain modest forces of the types such as these great powers possess.
As I stated at the beginning of this post, many of these units were formed for the Second World War, particularly the Marine and Airborne Divisions. Mountain/Alpine Divisions have existed since at least the First World War, and with the exception of a few units such as the Royal Marines' 3 Commando Brigade, are unmatched in their field. Motorized Divisions were formed for the Second World War, to secure ground that had been won by Armoured Divisions which (theoretically at least) were free to pursue the defeated enemy. Air Assault/Airmobile Divisions have only been around since the Vietnam War (although both the USMC and the French Army were important and successfull pioneers in this field prior to the US Army's first Airmobile division, the 11th Air Assault Division (renamed the 1st Cavaly Division [Airmobile] was formed and sent to Vietnam. As for Light Infantry Divisions, many that exist have been formed for the territorial defence of regions that possess extremes of terrain or climate and that preclude most or all use of Heavy ground units. Others, however, have been formed since the Vietnam War with a power-projection role in mind.
In general, such Light and Medium ground units have performed, and performed well, the roles assigned to them. That said, there are some problems and inconsistencies with many of them. I will deal with these by type:
1. Mountain/Alpine Divisions - Perhaps the oldest and most proven type of modern Light or Medium ground unit apart from the regular Foot Infantry Division itself, the Mountain Division (in competent hands) is probably unsurpassed in the role of teritorial defence of mountainous terrain. In the role of power-projection in mountainous terrain, it is also hard to beat. Only a few Commando units (albeit with additional and extensive mountain and also arctic warfare training and experience) such as the Royal Marines, may match or surpass this. Otherwise, the Mountain division seems secure in its role.
2. Marine/Amphibious Divisions - At present, only the USMC maintains Marines at a true Divisional level. That said, many countries possess nominal Marine Divisions (and more did so during the Cold War) as well as smaller formations, especially brigades. Marine units have typically performed quite well in the roles assigned to them. However, many of these Marine units are in fact something else.
The Royal Marines, while most certainly capable of amphibious assault, are in fact Commando Forces, as do appear the Russian Naval Infantry, the PLAN Marines, and a number of smaller Marine forces. While Marine Commando Forces tend to be more lightly equipped, organized, and supplied than regular Marines, their more extensive and advanced training suits them to conduct much more specialized and demanding tasks.
After all, regular Marine units are conventional infantry who possess additional training and equipement for amphibious assaults. Both US Army and US Marine Divisions conducted amphibious assaults in the Pacific during WWII (as well as some Commonwealth formations), and of course, in the largest amphibious assault of WWII, all eight of the divisions involved were drawn from the British, Canadian, and US Armies (with the first two armies reinforcing their divisions with full Tank Brigades, whereas US Army and US Marine Corps Divisions were typically reinfoced with only a Tank Battalion). Amphibious warfare training and equipment are certainly necessary, but as normandy proved, separate Marine Divisions are not necessary.
3. Airborne/Air Assault/Airmobile Divisions -
Soviet and German Airborne Divisions were formed for power-projection roles, not defensive purposes, and subsequently the British and US Armies followed suit. In the British Army, however, the Airborne Forces were initially intended to be fulled-fledged Commando Forces instead. Just as much of the amphibious role had fallen to the Army Commandos (and the Royal Marines were eventually converted en masse to the Commando role), so much of the airborne role had originally been intended to be taken on also by the Commandos.
However, the number of troops require for the Airborne role was judged to exceed the number of troops deemed likely to pass Commando training. In a survey of infantry battalion commanders, the British Army discovered that two-thirds of regular infantrymen would be capable of passing paratroop training; this, however, did not guarantee that the same men would pass the more rigorous and specialized commando training. The British Army, even with commonwealth and foreign volunteers to draw upon, never exceeded much more than around a dozen battalion-sized Commando units in existence at any one time. Clearly, even giving the entire Commando force parachute training would not suffice to supply entire divisions of paratroops.
No.2 Army Commando, the first commando fully trained in parachuting, was used as a cadre for the training and formation of the Airborne Forces, which recruited regular infantry for the Airborne role, raising 2 Airborne Divisions. The US Army created separate Ranger (Commando) Battalions (only about 6 at any time) and Airborne Divisions (5 in total). Although the Airborne Divisions performed well in their roles, the Commandos were all parachute-trained and fully Airborne-capable by 1944, and, formed into Brigades, spearheaded the Allied crossing of the Rhine River(the US Army 13th Airborne Division, which was scheduled to participate in the crossing by parachuting on the other side of the Rhine, never did so; its jump was cancelled, resulting in the unique distinction of its being the only US Army division of the war to never see battle). There has been no full-scale Airborne Division combat jump since 1945.
The Air Assault/Airmobile Divisions have had more mixed success. The first full-scale Airmobile Division to see combat was the 1st Cavalry Division in the Vietnam War. Helicopters gave the division great operational mobility, though once the infantry had disembarked from its helicopters, it reverted to regular Foot Infantry. Regular Foot Infantry Divisions also enjoyed, at times, some scale of helicopter support in the Airmobile role, though the cost and expense of the helicopters limited even the US Army to just two Airmobile Divisions (the 101st Airborne Division converting from the Airborne to the Airmobile/Air Assault role during the Vietnam War), and even then reducing to just one when the war wound down.
When applied to regions in which Heavy ground units, such as Armoured and Armoured Infantry forces typically operated, the Air Assault Division proved to be a poor performer. Exercises in southern Germany several years after the end of the Vietnam War demonstrated the unsuitability and vulnerability of Air Assault forces (specifically the 101st Airborne Division [Air Assault])to Heavy formations, particularly when one of its Air Assault Brigades was caught not once, not twice, but three times in the midst of landing its battalions by the same mechanized infantry brigade posing as a break-through enemy force that the 101st Airborne Division had to block and contain using its attack helicopters and helicopter-borne infantry. Although the attack helicopters ultimately halted the mechanized brigade (which lacked organic air-defenses at the time), the Air Assault infantry battalions were mowed down by the mechanized troops before they had a chance to react. Air Assault Divisions were thereupon excluded from use by the US Army in areas in which they would face Heavy ground units.
Similarly, the 101st Airborne Division was not committed to directly facing the Heavy ground units of the Iraqi Army during the first Gulf War, being used instead to establish forward operating bases and resupply points for the Heavy ground units of the US and British Armies which were used to deal with their Iraqi counterparts. Meanwhile, the 82nd Airborne Division was carried in trucks behind a French light armoured division to occupy an airbase in the deep desert and to take up a screening role on Third US Army's left flank. In the second Gulf War, the Iraq War, the US 3rd Mechanized Infantry Divisions and the reinforced US 1st Marine Division advanced on and took Baghdad, destroying the Heavy ground units of the Iraqi Army (those that showed up for the war, that is), while the UK 1st Armoured Division invested Basra. The 82nd and 101st Airborne Divisions engaged in mopping-up and occupation operations.
Airborne and Air Assault/Airmobile Divisions have seen only modest use since the Second World War, and indeed, in many armies regular infantry typically perform the Air Assault/Airmobile role (where and when suffcient helicopters are available), and Commando Forces have performed much of the Airborne role since late in the Second World War. The US Army Rangers, Belgian ParaCommando Regiment, 4th Battalion (Commando), Royal Australian Regiment (even though a paratroop battalion already exists in 3 RAR), and the German Army Special Operations Division (the regular paratroopers of 1st Airborne Division having given way to commando forces) are not only all parachute-trained, but are all Commando Forces, with training that goes well beyong that of regular Airborne troops. The Ranger Battalions, for one, have conducted as many combat airborne jumps since their creation in 1974 as have the rest of the regular US Army Airborne since the end of WWII. And as for The Parachute Regiment, their standards of training approach those of (and in a few areas, though not most, even exceed) those of the Royal Marine Commandos. If the roles of Airborne and Air Assault Divisions can be performed by Commando Forces even better (and in the case of Air Assault divisions even by regular infantry), it makes little sense to maintain them.
4. Motorized Divisions -
Motorized Divisions were designed for offensive purposes, specifically, to secure and hold ground that had been taken by Armoured Divisions. The Germans used Motorized/Panzergrenadier Divisions with great success for much of WWII. Once dug in, they had a fighting chance against enemy Heavy ground units; however, in the conduct of the mobile defence that the Germany Army was compelled to resort to later in the war, Motorized/Panzergrenadier Divisions suffered greatly at the hands of enemy Heavy units. Given both the shortage of, and the depleted condition of, the Armoured/Panzer Divisions that could take on enemy Heavy units, Motorized/Panzergrenadier Divisions were forced to enagage enemy Heavy units in mobile actions in lieu of proper Heavy units. Foot Infantry divisions that provided the base of the German defences were, given their near-total lack of armour or even motorized transport, completely unsuitable for enagaging in mobile operations to try to stop the very mobile enemy Armoured/Tank divisions.
Since the Second World War, the Motorized Division has seen relatively little action, and Motorized units have principally engaged in aid to the civil power type operations and counter-insurgency operations. Despite the conversion from trucks/lorries to armoured cars/wheeled APCs, Motorized Divisions remain vulnerable to even infantry anti-tank weapons, not to mention Heavy units. The utter lack of their use (despite their continued existence) since the Second World War in anything more serious than low-intensity warfare as well as their progressive replacement by tracked armoured vehicles in the guerrilla war in Afghanistan (where both their vulnerability to RPGs hampers their support to infantry and their wheeled suspension cannot bear the difficult terrain for extended periods of time, leading to cracks in the vehicles' hulls) testifies to their very limited utility.
Of the Light and Medium Ground Units that are presented here, it has been shown that only the Mountain Division, given its consistently satisfactory historical performance coupled with the unique and very specialized conditions of its mission, should probably be retained, although Commando Forces with proper training and equipment should also be capable of such a mission as well.
The roles of Marine/Amphibious Division, while quite historically have been quite satisfactorily performed by Marines, have also been so with regular Army Divisions, who moreover (in the case of British and Canadian Infantry Divisions that were each reinforced with a Tank Brigade for the amphibious assaults in Normandy, and again conducted amphibious assaults in the Netherlands) were throughout the following campaign. Marine Divisions, by contrast, when their operations are over, may or may not be withdrawn. As such, separate Marine Divisions are not necessary as regular infantry can do the same things with the proper training and equipment, and much of the more specialized amphibious missions are performed by Commando Forces such as the US Army Ranger and the Royal Marines anyway.
Airborne and Air Assault Divisions are similarly superfluous, as not only have Airborne Divisions not performed divisonal level combat drops since WWII, but the number of Airborne Divisions that existed in the US Army in WWII (5, of which 1 never saw combat) out of a total of 89 - a ratio of 1 AbnDiv to 18 other divs, as opposed to the 1 out of 10 now, demonstrates just how excessive even 1 parachute division is for the size of Army that even the US has. The PLA, with 3 Airborne Divisions and 3 more on the way, has never used even one in a combat parachute drop. Surely the US Army Rangers, suitably heavied-up a little (like the Royal Marines) could perform the US Army Airborne role even better. In Britain, the Paras are so close to the Royal Marines in many areas in terms of quality that it would be comparatively easy (strictly hypothetically speaking, setting aside all considerations of training staff availability and logistics, unit readiness, etc.) to fully convert them to Commando forces, with the substantial added edge that gives to infantry. This applies even more so to Air Assault/Airmobile Divisions, since their mission can be performed well enough by regular infantry provided they have the requisite helicopter support, given the sheer expense of such machines, and their limited quantity, it would make sense for the Air Assault mission to be performed by Commando forces. Again, it the US Army Rangers and commando-trained Paras would be much superior in this role to the 101st Airborne and their foreign counterparts.
Motorized Divisions are, in some ways, the most superfuous of all the Light and Medium groung units. Originally designed to follow-up behind Armoured Divisions, not only did they suffer badly at the hands of enemy Armoured Divisions, but as Armoured Divisions have acquired more Armoured Infantry themselves (late WWII German Panzer Divisions typically featured 2 infantry companies for every 1 tank company, as both offensive and defensive operations proved the utter necessity for Armoured Divisions to have much more Armoured Infantry than Armour itself), the Motorized Divisions have been relegated in the main to internal security operations. And even in those situations, when internal security operations give way to guerrilla war, Motorized Infantry may well find themselves replaced by Armoured Infantry, due to the latter's greater ability to cross difficult terrain and resist infantry anti-tank weapons.
Light Infantry Divisions have been left for last, and the verdict regarding them is split. Indeed, they have not even been treated with in the main body of this post, as this seems unnecessary given what shall follow here. As originally envisaged, Light Infantry Divisions, like mountain divisions, were designed for territorial defence in specific conditions of extreme climate or terrain which effectively restricted or even prohibited the use of Heavy ground units. They have traditionally (in competent hands) performed well in that role, even when heavily outnumbered and outgunned. The performance of the Finnish Army against the Soviet Army in 1940, which stunned the world, is the classic example of this. The subsequent performance of the Finnish Army during the Second World War, its ultimate outcome notwithstanding, proved that the performance of 1940 was not an anomaly, even if WWII provided much less opportunity for world admiration.
The other side of Light Infantry Division use is in the power-projection role. In this role, they have been largely unproven, except in "colonial" type small wars. The US Army's Light Infantry Divisions were intended to face enemy Heavy ground units, including Armoured divisions, in places such as the Middle East while buying time for US Heavy ground units to arrive. That they were never used in that role (even in the lead-up to the First Gulf War, for which they had in fact been intended), seems to indicate that even the US Army entertained real doubts as to their supposed capabilities. Critics who considered them to be "too light to fight" and much more difficult to supply entirely by airlift than their supporters maintained, seem vindicated. Only one Light Infantry Division remains in the active US Army (where there had been 4), the 10th Mountain Division, while the 25th Infantry Division is now half-Motorized. Given this, Light Infantry Divisions should only be used in the territorial defence role, in conditions of extreme climate or terrain, as they were intended to, and have been proven able to do.
In sum, except in conditions of extreme climate or terrain, most roles can be assumed by Heavy Ground Units (Armour/Armoured Infantry) or by Commando Forces. Only under the aforementioned conditions, should specialized Mountain, Light Infantry, Jungle Divisions be formed. Marine, Airborne/Air Assault, and Motorized Divisions are superfluous, and their roles can be performed just as well by a combination of Heavy Ground Units and Commando Forces.