Breaking the Mannerheim Line: Soviet Strategic And Tactical Adaptation in the Finnish-Soviet Winter War

Soviet dictator Joseph Stalin’s 60th birthday on Dec. 21, 1939, marked the nadir of the Red Army’s military campaign in Finland. A few weeks earlier, the party head of the Leningrad District, Andrei Zhdanov, had boasted he would personally deliver the signed documents of Finland’s surrender as his birthday gift to the vozhd or “leader,” as Stalin was referred to, on that special occasion. This was not to be. Rather, the last week of December was the time when Leningrad’s hospitals were overwhelmed by the huge number of casualties and the trains filled with wounded and dying conscripts coming from the Finnish front had to be send eastward all the way to Moscow. Many succumbed on this long, grim journey to their wounds. What had been initially thought of as a mere police action by the party apparatchiks — Stalin himself said that it would not take longer than two weeks — turned quickly into military stalemate for the Red Army. 

Four armies consisting of 21 Soviet divisions of the Soviet Leningrad Military District, a total of 450,000 troops, invaded Finland on the morning of Nov. 30, 1939 on eight main axes of advance along the 800-mile long Finnish-Soviet border without a formal declaration of war. Finnish troops, under the command of Field Marshall Carl Gustaf Emil Mannerheim, a former Tsarist officer, inflicted a stunning first defeat on the Red Army in the Battle of Tolvajäri on December 12. This was followed by an even more devastating defeat in the Suomussalmi and Raate Road battles, which took place across several phases of fighting between Dec. 11,  1939, and Jan. 8, 1940, and saw the wide-scale application of Finnish motti tactics (surprise attacks on the rear and flank of Soviet columns by highly mobile Finnish infantry designed to surround, immobilize and segment enemy forces, cut off their communications and supply lines, and eventually destroy them piecemeal), resulting in an estimated 22,000 to 27,000 Soviet losses, in addition to over 40 tanks and 270 other vehicles destroyed or captured. Other military disasters followed. Casualties in many Soviet divisions at the end of December ran as high as 70 percent. Soviet propaganda tried to explain away the setbacks by stating that the United States had sent 1,000 of its best fighter pilots to Finland and that the main defensive line of Finnish forces, the Mannerheim Line — an over 90-mile long system of bunkers (too few in numbers and far between to give mutual fire support to each other) and emplacements, protected by barbed wire, minefields, entanglements, and tank barricades and ditches — was stronger than the much-famed Maginot Line. 

In the end, however, the Winter War of 1939 to 1940 ended in a Soviet military victory, as did the follow on War of Continuation. What exactly enabled this costly victory, however, seemed to have escaped many contemporary military analysts at the time and many others in the profession ever since. With some obvious parallels to the ongoing war of Russian aggression in Ukraine, including massive intelligence failures — despite Soviet intelligence obtaining detailed plans of the Mannerheim Line in September 1939 but passing it on to the Soviet high command  — a series of severe military setbacks and defeats in the opening weeks of the war, an utter disregard for mounting losses of both men and equipment, poor logistics, rotten tactical leadership, and an inability to conduct combined arms operations, reexamining the Soviet experience in the Winter War may be timely. Perhaps the most important parallel between the two wars pertaining to this analysis is that both the Red Army in Finland and Russian forces in Ukraine have shown an ability to strategically, organizationally, and tactically adapt, underpinned by a determination to fight on (notably, neither the Winter War nor the war in Ukraine has so far seen mass desertion or capitulations), which makes both forces, despite serious setbacks and defeats, a dangerous enemy. As a result, one should indeed be wary of writing Russian forces in Ukraine off to early. The crucial difference between 1939 to 1940 and 2022 to 2023, of course, is the massive ongoing Western military support for Ukraine, without which the war may very well have gone the way of the Winter War.

Attrition and Combined Arms Maneuver

Finnish resistance has continued to draw admiration by commentators, military thinkers, and strategists over the decades since the end of the conflict, who continue to mine the Winter War for valuable lessons underpinned by the “natural tendency to root for the underdog.” Unsurprisingly, the war was also the focus of multiple wargames for NATO officers during the Cold War. With Russia’s ongoing war of aggression in Ukraine, the Winter War has gained even more prominence in discussions, with parallels drawn between the brave resistance of the Ukrainian people since Feb. 24, 2022 and the heroic stand of the Finns against impossible odds in the winter of 1939 to 1940. However, in many of these discussions, detailed analysis of the later phase of the war and the Soviet side are often neglected or rather cavalierly described. This has a long history dating back to the initial reporting and military assessments (e.g., the German Generalstab or General Staff) on the Red Army, which were primarily focused on the first weeks of the war. The general narrative is that the Soviet Army merely overwhelmed the Finns — who had run critically low on ammunition and supplies — with superior firepower indiscriminately applied and sheer numbers, while “the basic Soviet tactic did not change,” as one modern author put it.

This is incomplete at best: The Red Army, within weeks of suffering humiliating defeats at the hand of the Finnish military, was able to reconstitute itself, devise a new strategy, tactically adapt, and choose an operational approach that played to the strengths of Soviet forces, ultimately guaranteeing a victory that fell short of the Soviet regime’s initial political goals. Yet it was a military victory nonetheless. At the operational level of war, the Soviet high command chose a deliberate strategy of attrition to methodically wear down the Finnish defenders. At the tactical level of war, the successful adaptation of combined arms maneuver by Soviet ground forces proved to be the key to dislodge the Finns from their entrenched and fortified positions along the Mannerheim Line. What tied the operational and tactical levels of war together was the Soviet emphasis on massive firepower against an entrenched defender under circumstances where maneuver was exceedingly difficult to achieve their military-strategic objectives. The combination of the two, rather than Soviet quantitative superiority alone, ultimately led to the relatively swift collapse of Finnish resistance and the breaching of the Mannerheim Line in February 1940. 

Soviet Changes in Strategy 

The initial Soviet strategy devised by the commander of the Leningrad Military District, Kirill Meretskov, called for an overwhelming ground assault that would overawe the Finnish defenders, breach their defenders, and enable Soviet armored formations to conduct “deep battle” operations in the depth of the battlefield. In detail, this meant the 7th Soviet Army would break through the Mannerheim Line on the Karelian Isthmus in a frontal assault and then rapidly move on toward Helsinki, while the Soviet 8th Army was to support this attack north of Lake Ladoga and sweep southwards in a pincer movement landing in the rear of the Mannerheim Line defenses. Two other Soviet armies were to operate further north, but it was clear to both sides that the war would be decided on the Karelian Isthmus. This was not supposed to be a fire-heavy campaign. The later Soviet chief marshal of artillery, Nikolay Voronov, tasked with organizing ammunition supplies for the Soviet artillery, was told by his superiors at the time that his fire consumption estimates should be based on the campaign lasting no longer than 12 days. (Stalin insisted on a swift strike campaign lasting no longer than three weeks.) One reason for the war optimism sweeping some Soviet officers and the political leadership were swift Soviet victories over Polish forces and the Soviet defeat of the Imperial Japanese Army in Khalkin Ghol in 1939. Another was the belief, propagated by Andrei Zhdanov and other Leningrad communist party officials, that fifth columnists and large elements of the working class in Finland would revolt and help overthrow the government once the attack commenced.

Once it became evident by the end of December 1939 that the Red Army was incapable of conducting such a lightning combined arms campaign in Finland, the Soviet high command reevaluated the military situation. The reasons for the failure where manifold but included the severe weather — the subarctic winter of 1939 to 1940 in Finland was the second coldest on record in the country since 1828 — and a total lack of preparedness in the Red Army for such wintery conditions, and the terrain — forest after forest split by lakes, rivers, and swamps, with only a few unpaved roads passing through this inhospitable environment, making large-scale rapid troop movements difficult. The frontal assaults along the Mannerheim Line and other Finish defenses were stopped as a result. An earlier more conservative campaign plan, designed by Soviet Chief of the General Staff Boris Shaposhnikov, but dismissed by Stalin, was dusted off triggering an important switch in Soviet strategy in the first days of January 1940.

Most importantly, from an operational perspective, attempts at implementing Soviet deep battle doctrine, based on deep armored thrusts into the enemy hinterland, were all but suspended. In its place, the Soviets adopted a geographically more limited strategy of attrition with the narrow Karelian Isthmus as its center of gravity — in particular, around the town of Viipuri, a logistical key hub for the Finnish forces and the gateway to Helsinki. There, Soviet artillery would overwhelm the Finnish defenders of the Mannerheim Line with superior firepower, while armor in combination with infantry would continue to probe the defenses for weaknesses to exploit with larger armored formations held in reserve to support any breakthrough. 

To implement this strategy in detail and assume operational command, Stalin appointed Semyon Timoshenko along with Georgi Zhukov, the victor over the Imperial Japanese Army in Mongolia, as chief of staff on Jan. 7, 1940.  Timoshenko bluntly summarized his new attritional strategy the following way: “In frontal attack no enemy nor combination of enemies can hope to compare with us. By making a succession of direct attacks we shall compel him to lose blood, in other words to lose something he las less of than we have. Of course, we shall have enormous losses too, but in war one has to count not one’s own losses but those of the enemy.” This often-quoted public statement, however, stands in contrast to the actual Soviet strategy of attrition for this next phase of the war, which was primarily built around firepower — not human wave assaults. Timoshenko merely meant to signal to his foes in Finland that the Soviet Union would be willing to endure any price in blood to finish the war successfully. In the first week of January 1940, the newly appointed commander was given 25 days by the Soviet high command to prepare his force for new large-scale offensive operations in Finland. To give his troops some respite, the Soviet air force embarked on a 10-day long intensive bombing campaign to harass Finish forces and the civilian population while intense planning and preparations commenced. 

Soviet Organizational Changes 

Scrambling to ready his force on the newly created Northwestern Front (the Leningrad Military District name was abolished) for a new round of fighting at the beginning of February 1940, Timoshenko wasted no time instituting a number of organizational changes. He reorganized his force of 600,000 men on the Karelian Isthmus into two armies, the 7th Army (14 rifle divisions) and 13th Army (nine rifle divisions), and a reserve force of seven divisions under his direct command to exploit any potential breakthroughs. This force included five tank brigades, 15 air regiments, and a specially created force of 40 ski battalions and 200 ski squadrons. In total, 2,800 artillery guns ranging from 76.2mm to 180mm were amassed to support the effort. The Soviet high command also ordered the abolishment of the system of collective command, where a Soviet political commissar could overrule the decisions made by military commanders, facilitating more effective command and control. 

Additionally, Timoshenko took steps to boost the morale of combat troops. This included awarding medals to more than 2,600 veterans of the early battles of the war in mid-January (in total 50,000 awards for courage and merit including 400 Hero of the Soviet Union titles would be dispensed during the war). Through a reorganization of Soviet logistics, the supply situation of Soviet forces somewhat improved, although overall quality remained poor. However, as part of the reorganization in January 1940, Soviet soldiers were for the first time eligible to receive a ration of 100 grams of Vodka per day. “It warmed and cheered us during frosts, and it made us not care in combat,” remembered one Red Army soldier. Frontline units were also rotated and given days of rest and underwent tactical training in the rear. Furthermore, indoctrination and propaganda efforts were also stepped up not so much with party slogans but direct appeals to Russian patriotism and the motherland. On Jan. 24, 1940, Stalin also announced the creation of disciplinary NKVD battalions called control detachments with powers of life and death over Red Army soldiers and meant to discourage both desertion and retreat. Soviet sources claimed that both of these measures significantly boosted the fighting spirit of the Red Army. 

Soviet Tactical Changes 

Mannerheim compared Soviet tactics in the initial phase of the war to a “badly conducted orchestra.” However, this meant that all the instruments to form such an orchestra were there. Put otherwise, Timoshenko knew he had all the ingredients to form an effective fighting force that could break the Mannerheim Line, which consisted of a complex web of minefields, trenchworks, bunkers, minefields, and wire obstacles, supported by well-ranged direct and indirect fires, through complex combined arms operations. While Soviet officers dubbed the new tactical approach as essentially “gnawing through,” this belittles Timoshenko’s multi-faceted approach to improve Soviet doctrine. 

His basic attack scheme for the Mannerheim Line consisted of the following: a massive but well-coordinated artillery barrage that would precede a combined arms attack of armor, supported by infantry, with close air support held in reserve. Once the armor pierced the frontline — contrary to the incentives of Soviet Deep Battle doctrine, which was focused on achieving a decisive breakthrough via combined arms assaults, using reserves to exploit breakthrough in order to destroy enemy centers of supply and communication in depth — it would not outrun the supporting infantry and artillery and break into the rear. Instead, armor would wait and help expand the breach while follow-on waves of assault troops were sent in. If an attack was repulsed, the Red Army would simply shift its attack to another sector of the narrow Karelian Isthmus frontline, slowly grinding down Finnish defenses.

Timoshenko knew that the key to tactical success was effective coordination between assault troops — including both armor and infantry — and artillery units, as well as aircraft deployed for close-ground support. The reason for this was straightforward: Superior firepower alone had shown not to be able to breach the Finnish defenses. Infantry assaulting without armor and artillery support were quickly repulsed by energetic counterattacks from Finnish units, especially their tank-destroyer teams held in reserve, or by Finnish defenders holding out in well-prepared dugouts and bunkers. Without at least the ability to conduct rudimentary combined arms operations, there was the danger that this war would drag on for some time, with chances of Western intervention (at least in Stalin’s mind) ever increasing.

Consequently, as a first step to improve combined arms coordination, he attached to every assault formation a forward artillery observer equipped with new radio equipment. He also organized division-sized signals exercises to improve the integration of air and artillery fires. Furthermore, he instituted a rigorous training regime for the divisions leading the attack, facilitated by rigorous Soviet reconnaissance efforts to identify Finnish positions and strongpoints. For example, the Soviet 123rd Rifle Division staged three-full scale rehearsals in a life-size mockup of Finnish defenses of Lähde Road sector near the village of Summa, a key position in the Mannerheim Line and anchored by two bunker complexes (the “Millionaire” and “Poppius” bunkers), to practice storming the fortifications in coordination with tanks and artillery fire. All leading assault formations underwent similar training. Many of these large-scale exercises, which took place a few miles behind the frontlines, also included “live fire” phases. As a result of this intensive training regime, a rudimentary proficiency in combined arms operations was achieved in divisions spearheading the assault on the Mannerheim Line, although many deficiencies, especially in basic tactical skills and junior officer leadership, remained. Nonetheless, the training and rehearsals enabled the Red Army, despite its inflexible doctrinal confines at lower leadership levels and a general absence of self-initiative among junior and midlevel field officers, to function as a coherent combined arms fighting force at lower levels against stationary defenses in depths.

Breaching the Mannerheim Line 

As a result of Timoshenko’s restructuring, a Soviet rifle divisions sector on the frontline was reduced to one to one-and-a-half miles and heavily packed with artillery. The Soviet 123rd Soviet Rifle Division alone had 108 guns allocated to its attack sector focused on the “Millionare” and “Poppius” bunkers. The neighboring 100th Soviet Rifle Division, meant to seize the village of Summar, could count on an equal amount of fire support. Both divisions were part of the 7th Army tasked with breaching the Mannerheim Line on this 1.1-mile-long section situated in the so-called Viipuri Gateway — named after the strategically important town of Viipuri. The main Soviet attack in this sector was preceded by 10 days of intense bombardment from these guns and Soviet aircraft beginning on Feb. 1, 1940, with the Finnish defenders suffering an estimated 3,000 casualties. In a 24-hour period, more than 300,000 shells were fired. 

The main Soviet offensive against the bunker complexes and Summa was launched at 11:00 a.m. on Feb. 11, 1940.  

The 255th Soviet Rifle Regiment of the 123rd Soviet Rifle Division was, supported by two tank companies in its assault on the “Poppius” bunker. Soviet infantry and armor kept in close contact, and Finnish attempts to engage tanks with Molotov cocktails and satchel charges were repulsed. Soviet tanks ran up to the bunker and simply blocked its firing slits, forcing the Finnish defenders out in the open. Fierce close combat ensued. By 13:00, the Soviets had taken control of the bunker.  

Meanwhile, the assault of the 245th Soviet Rifle Regiment (of the same division) on the “Millionaire” bunker had gone less well. Lacking tank support due to marshy terrain, the regiment was tied down in the Finnish outlying trench system. A counterattack forced the regiment to withdraw. However, the successful attack on the Poppius bunker left the defenders in an isolated position. After fierce Finnish resistance and a number of failed counterattacks, the bunker finally fell to Soviet forces on either Feb. 12 or 13 (there is a discrepancy in Finnish and Soviet sources on the exact date). This was a key moment of the war. 

The capture of the two bunkers threatened the entire Finnish position along the Mannerheim Line. On February 13, Soviet forces also broke through a second line of Finnish defenses — a support line located about a mile to the rear of the main defense line — applying the same combined arms tactics. As the historian William R. Trotter writes in The Winter War: “The attacks followed a methodical pattern: strong artillery and air bombardment, followed by strong tank/infantry assaults.” He also pointed out that the Soviet infantry, charging in the open, were still willing to suffer enormous losses: “No matter how many men and vehicles were lost, the attacks would be repeated, in each division’s assigned sector, three, four, five times each day, with fresh Soviet units committed each time.” Finnish defenders at that stage were desperately low on ammunition, especially anti-tank shells, although most of their Swedish-made Bofors anti-tank guns were out of action due to the heavy Soviet bombardment or Soviet infantry swarming their positions anyway, and they had major difficulties maintaining their communication network, while casualties were becoming slowly but steadily unsustainable. 

More importantly, with the successful attacks on the bunker complexes preceded by a grinding artillery bombardment attriting its defenders, the Soviets had found a formula to break Finnish defensive lines, and it was just a matter of time before the Finnish position on the Karelian Isthmus would collapse. By the time the Finnish government decided to enter into peace talks at the end of February, an interim line of defenses anchored on the town of Viipuri had begun to collapse, and the road to Helsinki would soon be open to the Soviet invaders. The war would still go on for two more weeks, with heavy urban fighting taking place in the town of Viipuri. There, Finnish forces once again displayed their superiority in small infantry tactics as Soviet forces failed to eject all of the city’s defenders until three days after the signing of the Moscow Peace Treaty on March 12, 1940, marking the official end of 105 days of fierce combat in wintery Finland. 

Conclusion

The Red Army’s strategic and tactical adaption, while certainly noticed by the Finnish Army during the war, has received significantly less attention in post-war assessments than Finnish proficiency in small-scale infantry tactics, facilitated by sisu (guts), and general winter warfare skills which found its most legendary manifestation in the motti. One reason for this is that a lot of initial assessments were based on the performance of Soviet forces in the first weeks of the campaign rather than Soviet operations in February and March 1940. For example, the German Generalstab (General Staff) noted in an assessment on the Red Army in Dec. 31, 1939: 

In quantity a gigantic military instrument—Commitment of the ‘mass’.—Organization, equipment and means of leadership unsatisfactory—principles of leadership good; leadership itself, however, too young and inexperienced.—Communication system bad, transportation bad;—troops not very uniform; no personalities—simple soldier good natured, quite satisfied with very little. Fighting qualities of the troops in a heavy fight, dubious. The Russian ‘mass’ is no match for an army, with modern equipment and superior leadership.

The German Generalstab never revised its assessment of the Red Army following the second phase of the war and the Finnish defeat and, as Michael Kofman and Stephen Kotkin discussed, Nazi chauvinism undermined German strategy. Soviet officers held responsible for the initial failure of the attack, such as Colonel-General Vladimir Grendal, blamed the poor training and discipline of Red Army soldiers on the early defeats joining the chorus of those arguing that it was Soviet “mass” that ultimately was responsible for winning the war.

As I tried to show, this is simplistic at best. Within 25 days in January 1940, the Red Army not only was able to reconstitute and reorganize its forces and revise its campaign strategy but also develop rudimentary skills to execute complex combined arms operations with which the Mannerheim Line could finally be breached while maintaining fighting morale and unit cohesion. In a post-war conference conveyed by the Central Committee of the Communist Party in Moscow in 1940 to evaluate the performance of Soviet forces in Finland, the commander of a Soviet rifle division succinctly laid down the key evidence for military effectiveness as a result of this strategic, organizational, and tactical adaption: “The following circumstance is of paramount importance. In spite of difficult conditions, our troops seized this area. They overcame the enemy.” And this is what in the final analysis makes the difference between military victory and defeat. 

Looking for potential lessons from the Winter War for Ukraine today, a major takeaway is that a relatively untrained, tactically badly led, and ill-equipped conscript force can indeed strategically, organizationally, and tactically adapt under the right leadership. Such a force can achieve a level of proficiency at all three levels sufficient enough to ultimately militarily prevail against a more highly motivated, tactically superior (although the Finnish Army never mastered combined arms operations at scale and, while good at counterattacks, generally underperformed on the offense), if outnumbered and poorly equipped adversary. However, as one commentator put it: “[Russian President Vladimir] Putin has yet to find his 21st century Timoshenko” to facilitate such a rigorous top-down adaptation process in the Russian military currently fighting in Ukraine. It would be nonetheless a mistake to assume it cannot happen due to Russia’s military culture, inflexible leadership, mounting losses, and an adherence to rigid doctrine. Good leadership at the company, battalion, regiment, and even division levels was the exception and not the rule in the Red Army throughout the Winter War, and the Red Army still performed tactically poorly relative to Finnish forces. Yet the Red Army was still able to successfully adapt where it mattered. Russia’s military today is not the Red Army of the 1930s. By underestimating this force in 2023, however, we would commit the same mistakes many observers of the Winter War made of Stalin’s soldiers in 1939 and 1940. At the same time, it is important not to make the German mistake, which is learning the wrong lessons from an opponent’s failures, ignoring their adaptions and successes, and generalizing from the part of the war you like to validate your hubris rather than paying attention to what happened over the entire course of the conflict. 

Franz-Stefan Gady is a senior fellow with the Institute for International Strategic Studies. He can be followed at @hoanssolo. 

Image: Military Museum of Finland