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The American Army Infantry Division of World War II was a relatively compact unit, finely balanced between firepower and mobility, and with great care to keep the size of the supporting elements to a minimum, and a maximum number of troops on the line. It had three infantry regiments, a division artillery with three battalions of 105mm artillery, and one battalion of 155mm artillery. There were an engineer and medical battalion, a signal company, a cavalry troop, a quartermaster company, and ordnance company, and an MP platoon.

All of these assets were “organic,” that is, they belonged to the division, and were part of the Table of Organization and Equipment, spelling out just how they were to be, well, organized and equipped, and how they were to be manned.

But it was, especially in the later stages of the war, very common, indeed, almost habitual, for higher headquarters to augment an infantry division with additional assets to better accomplish its mission. Most Infantry divisions, especially during an attack, could count on receiving additional artillery battalions to reinforce the fires of the divisional artillery. And by the end of the war, virtually every Infantry division was supported by an attached independent tank battalion. The tank battalion was frequently further divvyed with a tank company supporting each Infantry regiment. So too, a tank destroyer battalion was very often attached, in much the same manner.

A brief digression on the organization of the Army at that time- the division was the largest organization in the Army that had a fixed TO&E. The higher headquarters of a division would normally be a corps, commanding from two to five divisions. Above the corps was the field army (that is, the numbered armies, such as Patton’s 3rd Army). An army normally commanded from two to five corps. 

A corps was a tactical headquarters. It had no organic assets beyond its own headquarters. It was assigned such divisions as it needed to accomplish its mission, and those divisions would cycle in and out of the corps as needed. The corps also had no services of supply, or other logistical assets. Those roles were the responsibility of the field army, and the field army pushed fuel, ammunition, rations, replacements, and parts forward directly to the divisions, as well as receiving casualties for evacuation to the rear.

The field armies, while not having a fixed establishment, did have not only organic services of supply, but also a great number of combat and combat support battalions. But the field army rarely directly exercised control over these battalions, instead attaching them to the corps and  divisions as needed to bolster their firepower.

For instance, GEN Omar Bradley, in his “A Soldier’s Story,” tells us that on the 24th of June, 1944, VII Corps, a part of his 1st Army, had assigned or attached four Infantry divisions, two Armored divisions, 20 independent artillery battalions, five independent tank battalions, 7 tank destroyer battalions, 11 anti-aircraft battalions, 8 Engineer battalions (and three independent Engineer companies), and two cavalry squadrons.  How those battalions were distributed throughout VII, Bradley doesn’t say, but that’s a pretty impressive total, considering the landings only began 18 days before, and that’s just one of four corps ashore in Normandy.

Let’s back up a bit, to World War II. As you doubtless are aware, incapacitating gas was first used on the battlefield, initially with crude chlorine gas, and later to other more deadly gases, such as mustard, in an effort to break the stalemate of trench warfare. It failed to accomplish that, and instead simply added to the already pretty horrific conditions at the front.

The first delivery method of these chemical weapons was to simply position cylinders of chlorine in front of friendly trenches, and wait until the wind would carry the gas across no-man’s land to the enemy trenches.

That was a very inefficient system, and quite hazardous if the wind unexpected shifted, and waiting for the right conditions was often impossible because of other tactical considerations.

Artillery shells that could deliver gas were developed (and used widely) but that distracted the artillery from the firing missions they were needed for.

The British developed the Stokes trench mortar, a simply steel tube with a firing pin at the bottom. It rested on a baseplate, and the tube was held at about a 45 degree angle by a simple bipod. It was fired by simply dropping a round down the tube. Something like a blank super shotgun shell at the bottom struck the firing pin, fired, and the expanding gases sent the round flying. The gas shell itself was a rather simple canister, with a fuze to burst it upon impact, spreading its chemicals in aerosol form.  It had a range of about 800 yards, more than enough to cover the distance across no-man’s land, and with its very high rate of fire, quickly became the preferred method of delivering chemical attacks.

After the war, while pretty much everyone was working to prevent or ban the use of chemical weapons in a future war, it was also recognized that a deterrent capability to retaliate in kind would be needed, lest any future opponent exploit a unilateral advantage.

In the United States Army, this responsibility fell to the Chemical Warfare Service , the forerunner to today’s Chemical Corps.

After World War I, the CWS began looking at improving upon the Stokes mortar as a delivery system for chemical weapons.  After a lengthy development, they (and interestingly, I mean they, not Ordnance) by 1928 fielded the M1 4.2” Chemical Mortar. One challenge CWS faced was the need for increased range. 800 yards had been fine in 1918, but something more was needed for the future, at least 2000 yards, and hopefully somewhat more. And because the chemical shells had no fins, a rifled tube was needed to keep the shells from tumbling in flight, reducing both accuracy and range. 

Further refinements of the mortar, now designated the M2,  eventually doubled the range to 4000 yards. It could fire mustard, lewisite, and, since the CWS was also responsible for battlefield smoke, a pair of smoke rounds, one firing a chemical compound known as FS that produced a cloud of thick white smoke, and the other, White Phosphorous, which also produced a thick white cloud of smoke, but was also a potent incendiary, and casualty producing agent, what with the intense heat of burning WP particles.

What there wasn’t, was a high explosive round.

And the reader will recall that in the 1920s and 1930s, the Army was incredibly small, and damn near penniless.  So, while the CWS had developed the M2, it hadn’t produced more than a handful of production examples. It wouldn’t be until the eve of America’s entry into World War II that the M2 would be produced in numbers, and fielded with troops.

With the expansion of the Army begun with the Selective Service Act of 1940, the CWS began to form Chemical Mortar Battalions. Originally, each CMB consisted of a battalion headquarters, and four companies, each with four firing platoon, with four tubes in each platoon. In 1943, the battalion lost one company, and each of the remaining three companies lost a firing platoon, bringing the CMB in line with the triangular organization of the ground forces.

It wasn’t until 1942 that MG William N. Porter, then head of the CWS, was able to secure permission to develop a high explosive round for the M2. And it wasn’t until March 1943 that GEN Marshall authorized the use of high explosive shells by the CMBs.

Too late for service in North Africa, the CMBs and their “four dueces” would first go into action July 10, 1943 during the invasion of Sicily with the 2nd Chemical Mortar Battalion.*  Prior to the landings, the entire battalion had only fired 35 high explosive rounds in training. Attached in support of the 45th Infantry
Division, 2nd CMB would fire some 35,000 rounds of HE during the 38 day campaign.  The M2 was popular with Infantry commanders because it was very quick to emplace, incredibly accurate, and could respond to calls for fire far faster than conventional 105mm artillery.

It wasn’t long before Infantry commanders were clamoring for more of the CMBs, and eventually a total of 25 would serve overseas during the war. It was standard operating procedure during both the Italian campaign, and in Patton’s 3rd Army, that every division in the attack would have a CMB attached, if at all possible.

Further CMBs would see service in the Pacific theater, and the Marine Corps would field 12 gun companies to support their infantry regiments.

The mortar itself, while simple, was not crude. It weighed 305 pounds, and could be broken down into three components for transport- the baseplate, the tube, and a monopod support. Originally, each gun section was supposed to be transported by two jeeps, one with the gun, and another with an ammunition trailer, but very often the guns were transported via a handcart over terrain too rough to be passable even by the legendary jeep.

The high explosive shell of the 4.2” mortar weighed just under 25 pounds, with an explosive filler of about 8 pounds of TNT. It could be fitted with either an impact fuze, or a time fuze for airburst, or a delayed impact fuze to allow the round to penetrate a bunker or similar target.  While it had only about a third of the range of a 105mm howitzer, the shell was quite comparable in its effect, and it had an incredible rate of fire, about 20 rounds per minute, per gun.

The range of the gun was, like many artillery weapons, adjusted not only by the elevation of the tube, but also by varying the charge used to fire it. When a round was uncased, it had a full charge. This consisted of a fixed base charge, and a series of fabric packets affixed to the base, and loaded with propellant. Consulting the firing tables would tell the appropriate number of charges needed for a given firing range, and the gun crew would simply remove and discard any excess charges needed.

The need to again increase the range of the 4.2” mortar led to a new design, the M30, with a completely revamped baseplate and support, a longer tube, and a range of almost 7000 yards. Introduced in service 1951, the M30 would equip the 2nd CMB in Korea.

Eventually, it was realized that the 4.2” mortar was, like the 60mm and 81mm mortars, more properly an Infantry weapon than a CWS weapon, and responsibility for it was transferred from the CWS to Ordnance.

And after the Korean War, the CMBs were stood down. But that hardly meant demand for the Four-Duece had gone away. Instead, each infantry unit would field its own firing platoons of the M30. Originally, this was a firing battery of 12 guns in the headquarters of the Battle Groups of the Pentomic Division TO&E (though, interestingly, manned by artillerymen). Later, with the reintroduction of the triangular division and conversion of battle groups back to battalions under the ROAD TO&E, each battalion had a six tube mortar platoon, manned by 11C Indirect Fire Infantrymen.

The 4.2” mortar would serve as the heavy mortar for mechanized infantry and armor battalions until well into the 1990s, until replaced by today’s 120mm heavy mortar.

Of note, the round appears to tumble in flight. First, our gunner is using a very, very slight charge, even less than the normal “fixed” base charge. Partly that is because these illumination shells are empty, and thus quite light, and partly that’s because they have a very small range to shoot on.

That in turn means there was a very low pressure in the chamber during firing, and the obturating ring likely failed to fully engage the rifling of the tube.

What’s an obturating ring, you ask? I’ll be happy to explain. Did you see the two discs he emplaced on the round after inserting the fuze, and before adding the firing charge? One was steel, and the other brass.

Because the M2 is muzzle loaded, the round has to be able to slide smoothly down the barrel to strike the fixed firing pin.

But any round that is able to slide smoothly down the tube won’t engage the rifling of the tube on the way back up, and thus won’t be stabilized.

When the round is fired, the steel ring is driven into the brass ring, flattening it, and increasing its diameter and forcing it into the rifling of the tube. This both allows the projective to be spun for stabilization, but also forms a seal that prevents gases from expanding past the projectile body, thus increasing the efficiency of a given charge.

Also known as a driving band, obturating rings are very common on most artillery and mortar projectiles, even smoothbore mortars.

*Sorry, Grump, I double checked. I was right, you were wrong.