In the first six months of Operation Barbarossa, the brutal Nazi invasion of the Soviet Union, Panzer tanks overran hundreds of miles of Soviet territory and reached the outskirts of Moscow before winter weather and reinforcements from Siberia brought a halt to their advance. In a period when the Red Army seemed on the verge of collapse, a lumbering forty-eight-ton heavy tank that could absorb German tank shells like so many spitballs was one factor that bought it badly needed time.

The Red Army was an early practitioner of mechanized warfare, with thousands of light T-26 and BT tanks in its operational units when Germany invaded. It also also developed huge T-28 and T-35 multiturret heavy tanks to punch through enemy defenses. However, these hulking “land battleships” proved ill conceived: they had great difficulty negotiating rough terrain, and their large hulls were surprisingly poorly armored.

RECOMMENDED: How America Would Wage a War Against North Korea 

Therefore, in the late 1930s, designer Josef Kotin rushed into production the simpler, more densely armored Kliment Voroshilov, named after the Soviet defense minister. The early-model KV-1s boasted an extraordinary seventy to ninety millimeters of armor, rendering them impenetrable to the standard thirty-seven- or forty-five-millimeter antitank guns of the day. By contrast, early war German Panzers ranged in armor from ten to thirty-five millimeters and weighed less than half that.

For armament, the KV-1 had a single short-barrel seventy-six-millimeter L11 gun in the turret, as well as 7.62-millimeter machine guns in the hull and turret. There was even a third machine gun in the rear of the turret to fend off ambushing infantry.

RECOMMENDED: North Korea Has 200,000 Soldiers in Its Special Forces 

The KV-1 debuted promisingly against the Finnish during the 1939–40 Winter War, with only one lost in action. The Soviets busily iterated, and the 1940-model KV-1s added higher-velocity seventy-six-millimeter F32 guns to that could bust Panzers with ease, and caked on even more armor. The KVs were organized into small sixteen- to twenty-two-vehicle battalions which served alongside T-34 mediums tanks in mixed brigades. The T-34 was cheaper and faster, had similar armament, and was also quite tough—but still not as tough as the KV-1.

The Soviets also produced over three hundred KV-2 tanks, which had a huge boxy turret mounting a massive 152-millimeter howitzer for swatting concrete bunkers. Though its shells could annihilate anything they hit, the fifty-seven-ton KV-2 was so unstable it couldn’t fire when on uneven terrain or while moving, and it did not reenter production after nearly all were lost in 1941.

Roadblock at Raseiniai

The Soviets Union only disposed of 337 KV-1s and 132 KV-2s in the Western military district when the Nazis invaded. German intelligence somehow failed to identify the threat they posed. However, the Wehrmacht should have had an inkling of its heavy tank problem from its experiences in the Battle of France, when Panzers faced heavily-armored Matilda II and Char B1 tanks that their guns could not penetrate. Unfortunately, the Allied heavies lacked the combined arms and logistical support to capitalize on initial battlefield success.

German commanders had also countered with heavy antiaircraft guns or howitzers in direct-fire mode to penetrate heavy armor. This was not ideal for an army on the attack, however, as the artillery had to be towed into position by soft-skinned vehicles and required minutes of setup time. Deploying them in front of advancing enemy tanks bristling with firepower was a risky business.

The shortcomings of this approach became evident in the Battle of Raseiniai, in the opening days of Operation Barbarossa. On June 23, German tankers in Czech-built 35(t) tanks were startled when KVs of the Second Tank Division rolled straight through their regiment, unperturbed by ricocheting thirty-seven-millimeter shells, to rampage amongst the infantry behind them. The 35(t)s had to wheel around and chase after the Russian heavies, eventually immobilizing some with hits to the tracks and driving off the rest.

However, one of the KVs penetrated far behind German lines before running out of fuel directly in the supply lines of the Sixth Panzer Division on June 24. When a fuel-and-ammunition convoy attempted to pass the seemingly abandoned tank, it opened fire and set twelve of the trucks ablaze. A battery of fifty-millimeter antitank guns began smacking the tank with high-velocity shells, but the Russian behemoth wiped it out it with cannon fire.

Then the Germans tried setting up a more powerful eighty-eight-millimeter flak gun seven hundred meters away to destroy the Russian tank, but it too was knocked out before it could land a shell on target.

That night, German pioneers tried sneaking up to the tank and blow it up with satchel charges. But as the sappers made a dash for the tank, its machine guns blazed into action, foiling the attack.

Perturbed that a single Russian tank had delayed an entire Panzer division for twenty-four hours, Gen. Erhard Raus coordinated a tank-infantry-artillery assault the following morning. The crew was stunned by repeated hits, including several penetrating shots from flak guns. German Schutzen finally climbed on top of the tank’s hull only for the turret to begin rotating! They hastily chucked grenades through some of the shell holes, bringing an end to the crew’s heroic stand.

Shock at Kamenevo

Despite the intimidating armor and firepower of the T-34 and KV tanks, the Wehrmacht seemed to roll on to victory after victory. After encircling and destroying over six hundred thousand Soviet troops in Kiev in September, the Germans finally committed to a drive on Moscow. The tip of the armored spear piercing northeast towards the Soviet capital was led by the Fourth Panzer Division’s Kampfgruppe Eberbach. This combined arms force included a motorcycle-infantry battalion, five companies of Panzer III and IV tanks, and nine towed eighty-eight- and 105-millimeter guns.

On the morning of October 6, 1941, the Kampfgruppe overran infantry and T-26 light tanks defending the town of Kamenevo, close to Mtsensk, 170 miles southwest of Moscow. However, as the Panzers rolled up the road leading to Tula, they were started to behold a vast armada of T-34 and KV tanks charging towards their left flank across an open plain. The night before, Col. Mikhail Katukov had hidden his Fourth Tank Brigade in an ambush position. The Soviet armor now pounced in a concentrated rush.

Heavy guns deployed on an overlooking ridge began picking off Soviet tanks, but there were too few to stem the tide. The armored horde overran the ridge, crushing several of the guns under their treads, and plunged into a wild melee with the undergunned Panzers. The only bright side for the Germans was at that short range, they had a slightly better chance of penetrating the thick Russian armor from the side.

Amidst the chaos of milling Russian tanks, Eberbach was eventually able to fall back, losing around a dozen tanks destroyed or damaged while knocking out eight of the Russian heavies—at least by their own count. The first winter snow arrived the following day. The Wehrmacht would secure the Mtsensk region three weeks later, but Katukov’s counterattack had fatally upset the pace of their advance, which fell just short of Moscow before the Russian winter froze it for good.

That victory came at great cost. The Red Army had lost nine hundred KV-1s and 2,300 T-34s by December 1941.

Unreliable Steel Beast

Through 1943 the Soviets built more than four thousand KV-1s, with successive model initially bolting on more and more armor to the overloaded vehicle. This trend peaked with model 1942 KV-1c, which boasted a ZiS-5 seventy-six-millimeter gun, a maximum of 130 millimeters of armor—and a road speed of just seventeen miles per hour! These assumed a major role in the Soviet offensives of 1942. There was also a run of around seventy KV-8s sporting flamethrowers in place of their main guns.

However, while the KVs were nearly invulnerable to early-war German tanks, they suffered from terrible visibility and defective transmissions. Frequent mechanical breakdowns and slow speed meant they struggled to keep up with the T-34s alongside which they served.

Finally recognizing these crushing defects, Soviet factories followed up with the lighter KV-1S, which selectively trimmed armor down to seventy-five millimeters, while fixing the transmission and vision slits. The more reliable vehicle could keep pace at twenty-eight miles per hour.

But by that time, new German tanks with improved armor and long-barreled seventy-five- or eighty-eight-millimeter guns could penetrate the KV’s formerly near-impervious armor. While the KVs were still tough, that no longer made up for their high expense and by then merely average firepower.

Thus, the KV-1 was fated to fade from prominence, unlike its stablemate the T-34. Only a few battalions of KVs served in the Battle of Kursk, the titanic armored clash of the summer of 1943. These proved completely outclassed in confrontations with new German Tiger and Panther tanks.

To counter that threat, Soviet factories churned out 148 KV-85s, a stopgap model upgraded with an eighty-five-millimeter gun. However, tank designers were already working on installing the gun on the T-34 tank, while the more heavily armed and armored Joseph Stalin series of tanks took over the KV’s role. Still, small numbers of surviving KV-1s would serve through the remainder of the war, including in the siege of Leningrad, the recapture of Crimea and the invasion of Japanese-held Manchuria in August 1945.