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What were assault guns?
Assault guns were one variety of tanks in WW2, common in the German and Red (Soviet) Army. The archetypical assault gun (Sturmgeschütz, StuG) was certainly the StuG III, which was based on the German tank Pzkw III. That tank was not prepared for mounting more powerful guns than 50 mm L/60 or 75 mm L/24 guns, but the good chassis became used in great numbers for a casemate tank (StuG III) with a 75 mm gun (the first few had a 75 mm L/24 gun before the Pzkw III got one, later and most StuG III had a capable 75 mm L/48 gun).
The Soviets used their own series of assault guns, most of them on T-34 chassis. They, too, mounted heavier guns than the turret tank version was capable of mounting. Some examples were the SU-85 (85 mm cannon, earlier than T-34/85), SU-122 (122 mm casemate howitzer), SU-100 (powerful and relatively rare 100 mm casemate cannon). Their heavier assault guns were based on the KV tank series chassis, mounting large calibre guns only.
Germany used the other chassis for assault gun-like tanks as well, but most of those types were assigned to Panzerjäger (tank destroyer) units - assault artillery units such as the Sturmgeschütz Abteilungen (assault gun detachments) were part of the artillery.
The British and U.S. Army followed very different approaches; the British had only the improvised and weird Archer tank destroyer, while the U.S. had a very different tank destroyer doctrine that preferred extra fast, thin-skinned tanks with open turret. They were meant as anti-tank vehicles, while assault guns merely morphed into that role.
The howitzer-equipped assault guns were mostly restricted to the infantry support role, though.
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The origin of the assault gun was in WW1. The German infantry needed guns with HE shells to defeat machine gun positions in the field, and used mountain guns as well as other relatively light guns for the purpose. The idea carried on; Germany, Soviet Union and Japan developed dedicated light (70-76 mm calibre, about 400-700 kg) and simple shielded guns for the purpose (and for indirect fire, a role later taken over completely by infantry mortars).
The problem with these infantry guns was that their mobility was too restricted. Their teams could quite easily get caught by competent enemy mortar troops and it was difficult to follow an infantry attack on rough ground with a crew-moved 500 kg gun.
The internal combustion engine and tracks as well as more armour plating were of course the solution, and exactly this was proposed in 1936 in the German army.
A few medium tanks were simplified (no turret, 75 mm L/24 casemate gun), took part in campaigns in 1940 and proved their value as a protected, mobile substitute for infantry guns.
The scarcity of German resources caused a conflict between the need for concentration on Schwerpunkt armour divisions and the scattered assault guns units for infantry of the line.
The assault guns batteries' great success and the great need of the infantry forces for such support coupled with the reduced vehicle price allowed for a decent quantity of assault guns. The organization of the assault guns helped as well; battalion-sized units (in practice less than 30 assault guns each) were held at higher HQs and assigned to support in infantry division sectors only on a as-needed (most) basis. This was more efficient than assault guns for every division; that would have been an unaffordable luxury.
The affordability and quality of assault guns later led to their employment in armour divisions as well, mixed with turret tanks with good effect.
Assault gun tactics
It's easy to learn about hardware basics, but tactics are the really interesting thing about assault guns.
The basic offensive tactic was to advance with (or behind) the infantry and lob 75 mm HE shells against enemy defensive positions and buildings in support of the infantry. More experience led to more advanced tactics, with a delicate balance between too close and too far infantry screens and even own escort infantry in assault gun detachments. The infantry-bound advance allowed for the use of experienced assault gun personnel scouts ahead in order to have a pair of eyes on the ground/terrain before any assault gun could get stuck on it.
It is interesting to see that much tank combat with main battle tanks after Desert Strom 1991 was quite similar to these offensive anti-personnel assault gun tactics (Croatia, Bosnia, Chechnya, Afghanistan, Iraq post-2003).
Offensive tactics such as pursuit or exploitation were no jobs for assault guns. They were simply not as good as tanks in these risky jobs. A dangerous enemy contact to the left or right of a formation could not as easily be countered because assault guns had their gun roughly pointed at the same direction as their movement. Turret tanks are in most terrains able to move in formation with some guns pointing let, forward, right and even to the rear. Assault guns had to turn and stop for effective firing. The necessity to point the front armour towards the enemy helped at times (and turning was quick with good gearboxes), but it stressed gear boxes, sometimes threw the tracks and it tended to let units deviate from their originally planned axis of advance. It was also more difficult to engage multiple targets in quick succession than with a turret.
These characteristics of casemate tanks (kind of similar to today's restrictions of tanks without full firing-on-the-move capability) were the price of the lack of a turret, but apparently overcompensated for by casemate tank advantages in WW2 if the proper tactics were used.
The assault gun on the defence was a different beast. Many tactics were possible, but the most interesting was apparently rooted in the fact that assault guns were operated by artillerymen.
The artillery had developed a far ambush tactic (Lauerstellung) in the last years before WWI (not sure about the exact timing, but most likely after the introduction of the first quick-firing cannons). A battery or half-battery was camouflaged and ambushed enemies on an open field with its destructive fire. This tactic was later adopted by heavy machine guns and almost fell into disuse among artillerymen. They did still know it, though - and employed the assault gun with this tactic against superior numbers of enemy tanks.
The StuG III of late 1942-1944 was very well capable of taking on enemy tanks on open terrain, but its employment in relatively small groups (batteries of 6-11 in theory, detachments of up to about 30 in theory - but more like a dozen in practice) helped to keep the necessity of superior tactics in everyone's mind.
Assault guns were also well-suited for a particularly interesting tactic that was and is very difficult to defend against: Tanks of all kinds can expose themselves for a very brief period, fire a shell against a previously identified and selected enemy position and return to concealment or cover. It's very challenging to defend against such a gradual wearing down of defensive positions. Even good anti-tank guns of WW2 and most modern anti-tank guided missiles of today can meet their limits in such a fight.
The assault gun as a 'cheap tank'
Infantry forces were no fast forces; the need for fast pursuit and exploitation was therefore low. Assault gun units were not required to be able to spearhead an attack into the enemy. The requirements for the hardware were therefore lowered, and this allowed for a cheaper tank.
(1) casemate gun tanks were acceptable, therefore no need for a turret (StuG III costed only 71% as much as a Pzkw IV with the same gun and similar armour protection)
(2) a weak side armour protection was acceptable (even normal tanks are usually weak on the sides)
(3) training requirements were lower, for combat was slower and the tactical repertoire a bit smaller than for turret tanks
Tanks are commonly characterized by the triad of firepower - protection - mobility.
Assault guns place a strong emphasis on firepower and frontal protection
Mobility and side protection aren't as important (the saved weight of a turret nevertheless allowed for a better StuG III mobility than Pzkw IV crew enjoyed.)
The most important requirement in regard to mobility was likely the ability to exploit many off-road firing positions and to support the infantry out of the line-of-sight of roads.
(Today's Stryker MGS was (is?) supposed to support infantry with a 105mm cannon, but it's not very off-road-capable and its gun traverse is no real substitute. It cannot be employed as an assault gun and needs to be used with different tactics that are more predictable because of the reduced cross-country mobility. It has a much greater machine gun firepower than WW2 assault guns as a plus.)
StuG III assault guns and similar German vehicles were relatively affordable, but certainly not poor vehicles. The odds of survival (or rather: remaining life expectancy) of assault gun crews were much better than for tank crews. Their kill ratios against tanks were excellent and superior to tanks of the same weight class. Even the ratio of total own losses (all causes) and knocked-out enemy tanks was very favourable. Their tactics were typically less risky than armour tactics, crews had a better chance of escaping to safety if their vehicle was knocked out. The smaller silhouette in comparison to tanks with comparable armour and gun added to the survivability.
Old main battle tanks as assault guns?
I mentioned the parallel between the limitations of a classic casemate gun tank and today's second-rate tanks (that are not fully capable of firing-on-the-move). The lower hardware requirements of the assault gun tactics usually don't exceed such MBTs' capabilities. The European countries have scrapped their second-rate MBTs, the Russians seem to be in the same process. There are (ten) thousands of such otherwise obsolete tanks all over the world, though.
Such old MBTs are usually considered as low-value tanks, or probably as mere cannon fodder in face of modern MBTs. This estimation stems in great part from the in many aspects very unfair battles against the Iraqi army in '91 and 2003. Such turkey shooting on often completely open terrain does not tell much about such old tanks, though.
Let's remember that the second-rate M4 Sherman tank was the standard tank of the U.S.Army for assault-gun-like infantry support in WW2 as well (despite being terribly outclassed by several other tank types in tank-vs-tank combat).
The effectiveness of old MBTs with assault gun tactics should be kept in mind!
Modern assault gun detachments?
The ideal assault gun unit has its own escort infantry with armoured personnel carriers. This is a necessity because not all normal infantry can be sufficiently trained in close cooperation (providing security) with assault guns.
Such an ideal assault gun unit is independent and to be temporarily assigned to light infantry brigades or battalion battle groups. This allows those light infantry outfits to remain light and only be reinforced with such armour support if necessary and on suitable terrain. We didn't need to optimize the efficiency of army formations that much during the Cold War, but it is a good idea for the 2010's with the expected downturn in military spending.
Today's "assault guns" would be superficially outdated main battle tanks - similar to the delegation of older tanks to secondary purposes in the German army during the Cold War (Leopard crowded out M48 to secondary purposes, Leopard 2 crowded out Leopard 1).
Such modern assault gun detachments could by default be assigned to Corps, either recovering or ready for temporary attachments.
Modern literature calls what's been done in the past two decades "tank-infantry cooperation" or similar - we could call it as well "assault gun tactics" (except that assault gun commanders were usually more careful*). At the very least we should open our eyes to the suitability and value of usually disesteemed older tank types for assault gun tactics.
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edit: P.S.: Something is not right with this article. I would have needed many more pages to describe assault gun tactics and make my point. Hopefully, all readers nevertheless get my point in this already quite long text; a reduced tactics set lowers the hardware expectations and in turn lets me think that disesteemed old hardware could be much more dangerous in the hands of competent users than is generally assumed.
*: Edited 2012
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