Contemporary study of the Battle of France showed that infantry without sufficient anti-tank weaponry were exceedingly vulnerable to armored forces; even if tank support was available, the Allied tanks of the day were mostly designed as infantry support weapons, and were incapable of fighting effectively against enemy tanks. The American armed forces, aware of the possibility of war against Germany, began to reorganize to meet this threat.
In April 1941, a conference focused on the future of antitank operations. The immediate effect was to create an anti-tank battalion in infantry divisions, but this organic anti-tank capacity was limited. The conference gave broad support to the idea of creating mobile anti-tank defensive units (commanded by corps or army formations) which could be deployed to meet an armored attack, but stalled on the issue of which branch of the Army should control it — the infantry (as a defensive role), the cavalry (as a mobile response force), or the artillery (as heavy guns). Interestingly, the armored branch did not press for control of the anti-tank units, feeling that it would be at odds with their general principles of maintaining the offensive. In May, General George C. Marshall cut through the knot by declaring it to be a sufficiently important issue to be dealt with as a combined-arms organization, forming an Anti-Tank Planning Board headed by Lt. Col. Andrew D. Bruce, and appointing Brigadier General Lesley J. McNair to take immediate action on organizing anti-tank forces.
Three anti-tank "groups" were quickly organized, with each of three anti-tank battalions drawn from infantry divisions and various support units, and tasked with "speedy and aggressive action to search out and attack opposing tanks before they had assumed formation". In August, a plan was laid out for a program of 220 anti-tank battalions.
The first nine units were deployed during the Louisiana maneuvers of 1941, equipped with towed 37 mm anti-tank guns and 75 mm guns mounted on half-tracks (the M3 GMC), and again in the Carolinas maneuvers that November. Their employment was judged a success — though not without some disgruntled voices in the armored branch arguing that the umpires had rigged the results — and on November 27, Marshall ordered the establishment of a Tank Destroyer Tactical Firing Center at Fort Hood, Texas, under Bruce, and the activation of 53 new anti-tank battalions under the command of General Headquarters. The term "tank destroyer" was used from this point on, as more psychologically powerful. On December 3, all existing anti-tank battalions were reassigned to General Headquarters and converted to tank destroyer battalions.
The new tank destroyer doctrine was formally stated in Field Manual 18-5, Tactical Employment, Tank Destroyer Unit, in June 1942. It laid down a focused doctrine — "There is but one objective of tank destroyer units… the destruction of hostile tanks" — and repeatedly emphasized an offensive, vigorous spirit. Tank destroyer units were expected to be employed as complete battalions, held in reserve and committed at critical points, rather than parceled out as small defensive strongpoints. The emphasis was heavily on mobility, and the ability to outmaneuver the attacking armor; this would feed into vehicle design, and cause the speed and power of a vehicle to be prioritized over survivability or — to a degree — firepower.
One side-effect of the new status of the tank destroyer force was that, as a fully-fledged independent branch, it was required by War Department policy to establish units manned by African Americans - the United States Army remained racially segregated until the Korean War, but the War Department placed pressure on the Army to ensure that a fair proportion of combat units were black. Two of the initially-converted battalions were black, with another four created in 1942 and four (of a planned six) in 1943. Several would eventually see combat, and one would become the first black unit to receive a Distinguished Unit Citation.
Critical analysis of the new force was mixed; while the 601st had successfully repulsed the attack, it had lost two-thirds of its strength in the process, contributing to Patton's declaration that the concept was "unsuccessful in the conditions of the theater". McNair responded by further clarifying the role of the tank destroyer — it was to be a highly mobile force, which sought to find and occupy favorable positions to engage an oncoming enemy attack. A tank destroyer "need only to maneuver for a favorable position, conceal itself thoroughly and ambush the tank — it was not intended to be used in a frontal offensive, or in assault in combat like a tank.
Three major changes to the overall doctrine emerged from the North African campaign. Firstly, the planned number of tank destroyer battalions was concluded to be excessive; the plan for 222 battalions was scaled down to 106, partly because of a reduction in the expected number of infantry divisions in the fully mobilized Army, but also because the expected massed German armored formations had not materialized. In October 1943, this would be further reduced to 78.
Secondly, flaws had emerged in the M3 and M10 tank destroyers — they were not fast enough, and had an overly high silhouette which made them vulnerable to direct enemy fire. General Omar Bradley, the commander of II Corps in North Africa, suggested that towed anti-tank guns could be reintroduced to infantry units — while it was feared this would lead to a lack of offensive spirit, it was undeniable that a towed gun could be dug in very quickly and efficiently, giving a very low-profile target. Bradley's proposal was not, however, intended to create towed TD battalions. His intent was to put the anti-tank assets in the hands of infantry units. The fact that the British Army had had success with dug-in antitank guns supported his proposal, and an experimental battalion was established in the summer with 3 inch guns. This quickly gained popularity — heavily supported by General McNair — and in November 1943 it was ordered that half of all tank destroyer battalions were to be equipped with towed guns; this was a major distortion of Bradley's suggestion.
Thirdly, the expected employment of the battalions shifted. While the formal doctrine still called for the employment of complete battalions, the Tank Destroyer Center began to emphasize a new focus on combined arms training and small-unit actions. New field manuals were prepared to discuss the independent operation of tank-destroyer platoons, and crews began to have specialized training in their secondary roles, such as indirect gunnery or anti-fortification work.
The second theater of operations for tank destroyer units was the Italian campaign, beginning with the landings in Sicily in July 1943 and continuing into mainland Italy that September. While German armor was present in the theater, it was rarely committed to battle in large groups; this was partly a result of the broken, heavily cultivated, terrain, but also due to a growing conservatism and defensive ethos on the German side. As a result, the tank destroyers found themselves used for a variety of other tasks, most commonly local artillery support.
A report in late 1944, reviewing the use of tank destroyer units in combat, found that in practice they were often expected to fulfill the roles of armor support:
When operating with armored forces, it was common to attach a company or a platoon to a tank battalion or company, and use them in an overwatch role. When operating in defense, tanks and tank destroyers were pooled as a rear-area reserve and brought into the line to blunt tank-led attacks against infantry positions.
A major lesson learned from the Italian campaign was that the conversion to towed guns was not as advantageous as had been previously thought. The mobility and protection of the tank destroyer was found to outweigh the towed anti-tank gun's low profile. At Anzio, a number of British towed anti-tank gun units were overrun and knocked out simply because they were unable to redeploy on short notice, while self-propelled guns were able to fall back and continue fighting.
The major tank destroyer used in Italy was the M10; though its gun was incapable of dealing with Panther tanks and Tiger tanks, it was more than efficient against most enemy armor encountered in Italy. The M18 was first deployed in the summer of 1944, and was not seen as a great success in the Italian theater; its high speed was only of limited use in the restricted terrain, and as a result it was effectively a slightly upgunned but substantially less survivable M10.
A revised version of Field Manual 18-5, introduced in June 1944, broadened the doctrine of tank destroyer operations. It allowed for a more dispersed deployment of the battalions throughout a force, and recommended that when enemy armor was only expected to be deployed in small groups, tank destroyers were to be distributed among forward units. It became general practice to attach a tank destroyer battalion semi-permanently to a division; this meant that it was locally available for emergencies, and that it would be able to train alongside "its" division when out of the line.
The most significant employment of tank destroyers in Normandy was in early August, at the battle of Mortain, where the 823rd Tank Destroyer Battalion (towed 3-inch guns) was on the defensive alongside the 30th Infantry Division. The division, which was in temporary positions and not prepared for a defensive engagement, was attacked by elements of four panzer divisions on August 6, under heavy fog. The 823rd put up a strong defense — knocking out fourteen tanks — but took heavy losses, being mostly overrun and losing eleven guns. This served to reinforce misgivings about the effectiveness of the towed units, and a report delivered to the Pentagon in December recommended they be phased out in favor of self-propelled units.
In December 1944 and January 1945, the Battle of the Bulge put American ground forces on the strategic defensive for the first time in Europe, as a German army group of 24 divisions (including ten panzer divisions with 1,500 armored vehicles) launched a major offensive in the Ardennes forest. The main thrust fell on two overstretched infantry divisions, both with attached towed tank destroyer battalions. Once contact was made, the towed guns were unable to reposition themselves or withdraw, and were often overrun by the enemy advance, or simply outflanked by infantry. The gun crews, unlike their self-propelled counterparts, had no protection against small-arms fire, and could easily be driven back by a squad of infantry. This lack of mobility was aggravated by the cold wet weather, and the rough conditions, which tended to bog down wheeled vehicles and immobilize fixed guns. Throughout the 1st Army as a whole, three-quarters of the tank destroyers lost were towed rather than self-propelled. One battalion, the 801st, lost 17 towed guns in just two days, while the M10-equipped 644th, which fought alongside it, found ideal opportunities for close-range ambushes and claimed 17 tanks in the same time period. It was clear that the towed guns had proved ineffective, and on January 11 1945, the War Office approved Eisenhower's request to convert all remaining towed units in the theatre to self-propelled guns.
Tank destroyers were to be found throughout the fighting in the Ardennes, fighting at close range in broken terrain much as they had done in Italy. The 705th, equipped with M18s, fought alongside the 101st Airborne at the siege of Bastogne, and played a key role in the defense of the town. The battle was also the first major engagement of the M36, with its 90 mm gun; the three battalions employed proved highly effective.
While the tank destroyers were broadly used in their intended role in the Ardennes — being used as a reserve to counter a massed armored attack — there were two significant differences between their use and the original doctrine. Firstly, there was no central strategic reserve — most tank destroyer battalions were assigned to divisions and kept near the front line, rather than massed in the rear. Secondly, the battalions were rarely committed as a whole; as had become common, they were effectively used as local anti-tank assets, with a platoon or a company assigned to an infantry battalion to bolster its defensive strength.
After losses in the Ardennes, German armored capability in the West had effectively collapsed, both through combat losses and through logistical limitations. As such, the tank destroyer battalions spent the closing months of the war as mobile support units, parceled out into secondary roles.
The US Army finished the war with 63 active tank destroyer battalions, mostly self-propelled.
While tank destroyers had proven their versatility and efficiency in combat, especially in secondary roles, their long-term utility was becoming doubtful by 1945 in light of changes to Army doctrine. Their primary role was to destroy enemy armor, but this role was being usurped by tanks, as had already happened in many other armies. The most powerful tank destroyer to be fielded, the M36, mounted a 90 mm gun; the same armament was carried by the M26 Pershing heavy tank, which was beginning to reach front-line units by the end of hostilities. The M26 was redesignated as a medium tank shortly after the end of the war, becoming the standard vehicle of armored units, and further reducing the need for any specialist anti-tank capacity. In effect, tank destroyers were used just like tanks in many cases. Study of ammunition expenditures shows that tank destroyers in Europe fired about 11 rounds of high-explosive (HE) ammunition for every round of armor-piercing (AP) ammunition, showing conclusively that they were used for general support duties far more often than as anti-tank assets.
In addition, the Army was reducing fast after the end of hostilities; the tank destroyer branch cost the equivalent of three or four full divisions, a definite luxury for a non-essential service. On November 10 1945, the Tank Destroyer Center was closed, effectively ending the long-term prospects of the force, and the last battalion was inactivated by 1946.
Three initial organizations were laid down in December 1941; two were light organisations equipped solely with 37mm guns, and were abandoned as soon as possible to standardise on the third type, modelled after the 893rd Tank Destroyer Battalion, and officially declared the standard organization in June 1942, with the publication of FM 18-5. This was designated the "tank destroyer battalion, heavy, self-propelled", and consisted of a headquarters company, a reconnaissance company (including a pioneer platoon), and three tank destroyer companies of twelve vehicles each; these three companies each had one platoon of 37mm self-propelled guns and two platoons of 75mm self-propelled guns. Each platoon had two sections of two guns each, an anti-aircraft section of two self-propelled 37mm guns, and a "security section" of twelve infantrymen. This gave a total of twenty-four 75mm guns, twelve 37mm guns, eighteen anti-aircraft guns, and 108 security troops. The vehicles used were the M3 GMC, with a 75mm gun, and the M6 GMC, a 3/4 ton truck with a 37 mm gun mounted in the rear compartment.
The battalions deployed to North Africa used this organization, with both heavy and light anti-tank guns, but as reports from Europe indicated that light anti-tank guns were no longer playing a significant role in combat, the light platoons were converted to a third "heavy" 75mm gun platoon under a new organization issued in November 1942. As M10 tank destroyers came into use, with a 3-inch gun, these replaced the 75mm gun-equipped M3s.
By 1943, the role of tank destroyer battalions was becoming better understood, and the early ideas of mass employment of tank destroyer units had become obsolete. As a result, the number of battalions planned was steadily reduced, and the manpower and equipment assigned to them was reduced. A new table of organization produced in January 1943 reduced the overall manpower by 25%, by eliminating the anti-aircraft units (which had proven unnecessary), reducing the numbers of supporting units, and combining the platoon headquarters with the security section. There was no reduction in actual combat strength.
In early 1943, stemming from problems found in North Africa, the Army began to consider a more defensive role for tank destroyer units. After trials in January, fifteen battalions were ordered to convert to towed guns on 31 March, and shortly thereafter it was decided that half of all tank destroyer units would convert to a towed organization. The broad outline of the battalion remained - three companies of three platoons of four guns - but the reconnaissance company was downgraded to two platoons in the headquarters company. However, the need for larger gun crews and more security troops meant that the manpower of the unit increased again. These units were equipped with 3-inch M5 guns towed by trucks or M3 Halftracks. However, combat experience in Normandy and Italy - coupled with the fact that purpose-built M18 and M36 tank destroyers were becoming available - showed that the towed units were underperforming compared with self-propelled ones. After the Ardennes campaign, where 85% of all tank destroyer losses were towed guns, it was decided to convert all towed units back to self-propelled guns.
Some early units were equipped with towed 37 mm guns, but this was abandoned as soon as possible. In addition to the 75 mm-equipped M3 GMC, the original attempt to produce a tank destroyer, a vehicle was developed utilizing the common 37mm anti-tank gun. This was designated the M6 GMC, and was simply a 3/4 ton truck with a 37 mm gun mounted in the rear compartment. A large number were produced, equipping one platoon in each company, but this was abandoned after November 1942 and the vehicles exchanged for M3s. The M6 saw only limited service in North Africa, and was declared obsolescent in 1943. A few were used by Free French forces in Europe in 1944-45, and the 37mm gun mounts were added to a few M2 halftracks at the unit level.
The deficiencies of the M3 were quickly noted in the 1941 maneuver, and the experience gained allowed a "perfect" advanced tank destroyer to be planned, one which would give a high priority to speed. However, it would take some time to be developed, and a simpler design was looked for as a stopgap. It was decided to take the proven M4 Sherman chassis and give it a modified 3 inch high-velocity anti-aircraft gun; the resulting combination was designated the M10 Gun Motor Carriage (GMC), later nicknamed the "Wolverine". Compromises were made to the vehicle; the turret was left open and lightly armored, keeping the weight low in order to prioritize the speed of the vehicle. A small number were used in North Africa, and it became a standard vehicle thereafter.
The experience of employing the M3, M6 and M10 in North Africa all fed into the plans for the next generation tank destroyer, which eventually saw service as the M18 GMC, nicknamed the "Hellcat". It was equipped with a newly designed 76 mm gun — firing the same shell as that on the M10 — mounted on an all-new chassis. This new design allowed it to be ten tonnes lighter than the M10, which allowed for a remarkable road speed of over 50 mph. However, it was still lightly armored — indeed, it had only half the armor of the M10 — and suffered many of the survivability problems of its predecessors. The M18 began to see service in mid-1944.
The final tank destroyer to enter service was the M36 GMC, nicknamed the "Jackson". This was a development of the M10 hull with a large turret mounting a 90 mm high-velocity gun, the most powerful vehicle weapon which would be carried by American forces in Europe. A prototype was originally constructed in 1942, as an experiment, and the design was standardised in June 1944. In July, the European command requested that all M10 battalions be converted to M36s, and the first vehicles reached the front lines in September. It proved more than capable of countering heavy tanks - it was recorded as disabling a Panther tank at 3,200 yards - and its roots in the M10 meant that it possessed greater survivability than the M18.