The 'Vyborg Offensive Operation' was the Soviet larger and more southerly of the two offensives together constituting the 'Vyborg-Petrozavodsk Offensive Operation' (10 June/25 July 1944).

The 'Vyborg Offensive Operation' itself comprised three basic elements as the 'Vyborg Offensive Operation' proper of 10/20 June, the 'Viojoki-Laappeenranta Offensive Operation' of 21 June/15 July and, as a sub-element of the latter, the 'Koivisto Landing Operation' of 20 June/25 July.

Otherwise known as the 'Karelian Offensive Operation', the 'Vyborg Offensive Operation' was a strategic operation by General (from 18 June Marshal Sovetskogo Soyuza) Leonid A. Govorov’s Leningrad Fronts against the Finnish forces of Kenraaliluutnantti Karl Lennart Oesch’s Isthmus Forces (Kenraalimajuri Hjalmar Siilasvuo’s III Armeijakunta on the left and Kenraaliluutnantti Taavetti Laatikainen’s IV Armeijakunta on the right with Kenraalimajuri Antero Svensson’s V Armeijakunta farther to the rear) on the Karelian isthmus during the later stages of the 'Jatkosota' continuation war on the Eastern Front. The Soviet forces captured East Karelia and Vyborg, but the campaign on the Karelian isthmus then reached a stalemate.

In January 1944, Soviet forces raised the siege of Leningrad in 'Iskra' and drove Generalfeldmarschall Walter Model’s (from 31 March Generaloberst Georg Lindemann’s) Heeresgruppe 'Nord' back to the line linking Narva, Lake Ilmen and Pskov in the 'Leningrad-Novgorod Strategic Offensive Operation' and 'Narva Offensive Operation'. Finland asked the USSR for peace terms in February, but the Finnish parliament then considered the resulting Soviet terms too harsh for fulfilment. After Finland had rejected the peace conditions, and the German retention of northern Estonia denied the Soviets the use of the Gulf of Finland and thus the Baltic Sea, the Stavka started to prepare an offensive to force Finland’s exit from the war, allow access to the Baltic Sea via Finland’s coastal waters and, most probably, the the conquest of Finland.

The result of the planning was the 'Vyborg-Petrozavodsk Offensive Operation', which was to be based on a two-pronged offensive, one from Leningrad through the Karelian isthmus via Vyborg to the Kymi river, and the second farther to the north across the Svir river through Olonets Karelia and Ladoga Karelia, in the process taking Petrozavodsk and Sortavala, past the 1940 Finnish/Soviet border, as the first step in an advance deep into Finland and outflanking the Finnish prepared defences on the Karelian isthmus. The Soviet plan called for the main strength of the Finnish army to be destroyed on the Karelian isthmus, and the remnants of the Finnish army to be driven against the western shore of Lake Ladoga between the two assaults and Lake Saimaa. As such, the main strategic objectives of the Soviet offensive were to drive the Finnish forces away from the area to the north of Leningrad, to drive Finland out of the war, and to bring about better conditions for a major offensive to the south against the German forces.

The Finns had been preparing defensive fortifications since 1940, and as a result there were three defensive lines on the Karelian isthmus. The first two were the 'Päär-linja' (main line) built along the 1941 front line, and the 'VT-linja' (Vammelsuu-Taipale line) parallel with the 'Päär-linja' but some 12.5 miles (20 km) to its rear. Sited to make maximum use of the area’s defensive terrain and reinforced with numerous concrete fortifications, the lines were essentially very effective, but their construction had not been completed by the summer of 1944. The third line was the 'VKT-linja' (Viipuri-Kuparsaari-Taipale line), was still largely in the planning stage, and work on the construction of the fortifications began at a time late in May 1944 at the Vyborg western end of the line. Farther to the north-east, on the northern bank of the Svir river, the Finns had prepared an area of defence in depth studded with strongpoints based on concrete pillboxes, barbed wire, obstacles and trenches. After the 'Talvisota' winter war, the 'Salpa-linja' had been built behind the 1940 border, with concrete bunkers in front of the Kymi river, as a bunker line on the eastern border. Built between the end of the 'Talvisota' and start of the 'Jatkosota', the 'Salpa-linja' was designed to defend Finland against a renewed Soviet invasion, and was some 750 miles (1200 km) long between the Gulf of Finland and Petsamo on northern Finland’s Arctic Ocean coast.

To overcome these obstacles, the Stavka assigned an additional 11 divisions and nine regiments of tank and assault guns to the Leningrad Front, whose strength on the Karelian isthmus was therefore 19 divisions, two fortified regions of divisional strength, two tank brigades, 14 tank and assault gun regiments and almost 3,000 pieces of artillery and multiple rocket launchers (more than 220 batteries). Some 1,500 warplanes of General Leytenant Stepan D. Rybalchenko’s 13^th Air Army and the air arm of the Baltic Fleet also contributed to the operation, which also benefited from the commitment of some surface warships and naval infantry units of the Baltic Fleet.

To the east of Karelia, the Stavka allocated to General Kirill A. Meretskov’s Karelia Front an additional nine divisions, two engineer brigades, two tank brigades and thee assault gun regiments, raising its strength to 16 divisions, two fortified regions, five independent infantry brigades, two tank brigades, three assault gun regiments and three tank battalions. The front was supported by Ladoga and Onega Naval Flotillas and General Leytenant Ivan M. Sokolov’s 7^th Air Army.

On the Karelian isthmus front the Soviets deployed an average of 193 pieces of artillery per mile (120 pieces per km), with as many as 354 pieces of artillery per mile (220 pieces per km) on the breakthrough sector at Valkeasaari. In addition to the heavy coastal artillery of the Leningrad area and the guns of the Baltic Fleet’s most powerful warships (battleship Oktyabrskaya Revolutsiya and heavy cruisers Kirov and Maksim Gorkiy), the Stavka had also assigned a number of heavy siege artillery pieces, in the calibre range between 11 and 12 in (280 and 305 mm) to support the attack.

On 9 June, one day before the main Soviet offensive, the 1,600-strong 13^th Air Army undertook a major air attack and at the same time artillery units of the Leningrad Front and the heavy guns of the Baltic Fleet’s warships shelled Finnish positions for 10 hours. The weight of the Soviet air and artillery onslaughts surprised the Finns, despite the fact that they occupied well-fortified positions, and had an adverse effect on their morale, with the result that several Finnish units retreated and many Finnish soldiers deserted.

On 10 June, General Leytenant (from 18 June General Polkovnik) Dmitri N. Gusev’s 21^st Army, with General Leytenant Nikolai P. Simoniak’s XXX Guards Corps in the van, started the 'Vyborg Offensive Operation' on the Valkeasaari sector, which was held by the 1^st Regiment of Eversti Jussi Sihvo’s (from 17 June Eversti Kai Savonjousi’s 10^th Division. During the day, the Soviet formations and units took many of the Finns' forward trenches and destroyed fortifications, shattering the Finnish initial defence line along the breakthrough sector. On 13 June the 21^st Army reached the partially completed 'VT-linja', and within two days had breached this at Kuuterselkä. Despite the fact that the first two defence lines had been breached, however, the Finns rallied and their resistance then managed to delay any further Soviet advance.

Over the same few days, General Leytenant Aleksandr I. Cherepanov’s 23^rd Army attempted to break through at a perceived weak point in the 'VT-linja' at Siiranmäki. Although the Finns succeeded in containing the Soviet breakthrough at Siiranmäki, this was not sufficient to allow the 'VT-linja' to be held as the sector round Kuuterselkä had already been breached. In the fighting around Siiranmäki against the 7^th Regiment between 13 and 16 June, General Leytenant Georgi I. Anisimov’s LXXXXVIII Corps suffered 3,784 casualties including 887 men killed.

The Finns now sought to buy time through the use of delaying tactics as they fell back, the object being to provide an opportunity for reinforcements from the East Karelia front to reach the isthmus and for the 'VKT-linja' to be readied for combat. On 19 June, however, the first formations and units of the Leningrad Front had reached Vyborg (Finnish Viipuri), and the Soviets completed the first phase of their offensive when they took this city on 20 June as the defenders of Eversti Armas Kemppi’s (from 22 June Eversti Yrjö Sora’s) 20^th Prikaati (brigade) fled in panic. Though the Leningrad Front had thus succeeded in taking Vyborg within the schedule set by the Stavka, it had been unable to prevent the retreating Finnish units from regrouping on the 'VKT-linja'.

By this time Suomen marsalkka Carl Gustaf Emil Mannerheim, the Finnish commander-in-chief, had asked Germany for assistance, and on 17 June the warplanes of Oberstleutnant Kurt Kuhlmey’s Gefechtsverband 'Kuhlmey' started to arrive in Finland, followed on 21 June by the first elements of Hauptmann Hans Wilhelm Cardeneo’s 303^rd Sturmgeschützbrigade (at half strength) and Generalmajor Hero Breusing’s 122^nd Division. New-generation German anti-tank weapons, the Panzerfaust and Panzerschreck, were also issued to Finnish troops.

On 21 June, the Stavka ordered continued attacks on the Finnish defence line linking Imatra, Lappeenranta and Virojoki on the 'Salpa-linja' in the 'Virojoki-Lappeenranta Offensive Operation'. Another Soviet group was also to attack to the north at the same time toward Käkisalmi (now Russian Priozersk) in order to surround the Finns defending the eastern part of the 'VKT-linja' as preparations were being made for an advance toward Kotka, Kouvola and the Kymi river.

On 21 June the Finnish government asked the USSR for peace terms, but the reply received on the following day demanded a complete Finnish capitulation before any conditions could be presented. This created divisions in the Finnish government, but Mannerheim stated that this could be construed only as a demand for unconditional surrender, and was rejected.

With reinforcements, there were now 268,000 Finns with 2,350 pieces of artillery, 110 tanks and assault guns, and 250 warplanes with which to oppose two Soviet fronts. Some 40% of the Finnish men and guns, as well as all the armour, was on the isthmus. In overall terms, the Soviets had a 1.7/1 advantage in men, 5.2/1 advantage in artillery, and 6/1 to 7/1 advantage in warplanes and armour over the Finns, who had 14 infantry divisions (at 12,000 men each), one armoured division (at 9,200 men), five infantry regiments (at 5,100 men each), one cavalry regiment (at 4,300 men), seven independent border Jäger battalions, coastal defence forces, and headquarters and corps artillery units. Fewer than 40 Finnish tanks and assault guns were modern and fewer than 100 of the warplanes: this gave the Soviets a matériel advantage in the order of 20/1 in the middle of June.

The campaign continued on 25 June as the Soviets breached the 'VKT-linja' at Tali, between Vyborg Bay and the Vuoksi river. On the following day, it became clear to the Soviets they they would not be able to effect a breakthrough at Ihantala, the Leningrad Front attempted a double envelopment with the twin assaults in Vyborg Bay and Vuosalmi. However, the Finnish army was able to hold its positions on these sectors of the front. On 12 July the Stavka ordered the Leningrad Front to release a number of offensive elements from the Finnish front for redeployment, and on 15 July the Soviet forces were ordered to go over to the defensive as offensive elements (mostly of armour) were transferred to the German front for use in 'Bagration'.

According to captured reports, the strength of Soviet regiments and battalions, especially in the Ihantala and Äyräpää areas, suggested that they had suffered very heavy losses. It had been estimated that 10 Soviets divisions each had fewer than 2,000 men, and the losses were notably heavy in the guards divisions.

Even so, the Soviets tried to penetrate deeper to the west after Battle of Ihantala, most notably in the Äyräpää area until 18 July and on the Karelia Front right into the early part of August. All the Soviet offensive efforts in fact continued until very end of the campaign despite the fact that there was no change of effecting any decisive breakthrough.

After the Soviet offensive on the Karelian isthmus had pushed to the north-west past Koivisto, the Finnish forces defending the islands in Vyborg bay became isolated. After its 21^st Army had failed to attack and seize these islands, the Leningrad Front allocated the 'Koivisto Landing Operation' to Vitse-Admiral Vladimir F. Tributs’s Baltic Fleet. The initial Soviet landing on 20 June was contained, but the Finnish navy soon evacuated the defending forces in a movement which was largely unopposed. Although the willingness of the local commanders to order the evacuation did serve the purpose of preserving the defending forces, the loss of the islands proved costly as it provided the ships of the Baltic Fleet with a safe route to the Vyborg bay.

After capturing Vyborg, the leading elements of the 21^st Army tried to push forward along the main road to the north from the city, but the availability of strong artillery support, the narrow nature of the area of operations, and the presence of terrain favouring the defence made it possible for Everstiluutnantti Alpo Marttinen’s 61^st Jalkaväkirykmentti (infantry regiment) to check the Soviet advance, thereby forcing the 21^st Army to find a more suitable location for breaching the 'VKT-linja'.

On 22 June the 21^st Army met stubborn Finnish resistance at Tali and was brought to a halt. After bringing up fresh troops, the 21^st Army then managed to push Finnish line back to Ihantala, but failed to make any breakthrough. The resulting Battle of Tali-Ihantala is generally considered to have been the largest battle in the history of the Nordic countries history. The 23^rd Army joined the offensive by attempting to break through the Finnish line between Tali and Vuoksi toward Noskua, but the repeated Soviet attacks were halted by the highly efficient Finnish artillery. In the end the battle of Tali-Ihantala between 25 June and 9 July was a defensive strategic victory for the Finnish army despite its losses of 8,500 men (1,100 killed, 1,100 missing and 6,300 wounded) to the Soviet losses of about 4,500 to 5,500 men killed and 13,500 to 14,500 wounded, as well as the loss of 300 tanks and between 120 and 280 aircraft. The Finnish victory removed all possibility of the Soviet forces breaking through into the Finnish heartland and securing the road to Helsinki. Soviet losses peaked on 28 June, when the Leningrad Front lost more than 5,000 men including 1,800 killed.

After the initial effort of the 21^st Army in the Battle of Tali-Ihantala had not produced a breakthrough, the Leningrad Front ordered General Leytenant Ivan T. Korovnikov’s 59^th Army to capture the islands dominating Vyborg bay and thence undertake an amphibious landing on the mainland opposite them. Although the landings on the islands were ultimately successful, between 30 June and 10 July, the attempted crossing to the mainland failed.

On the mainland, as the 21^st Army was unable to advance, the 23^rd Army attempted a crossing of the Vuoksi river and lake system in the area of Vuosalmi between 4 and 17 July. Although the Soviet forces succeeded in pushing the Finnish troops from the Äyräpää ridge dominating the crossing and managed to establish a strong bridgehead on the opposing side, the defending Finnish troops were able to contain it. In this Battle of Vuosalmi, otherwise known as the Battle of Äyräpää-Vuosalmi, the Soviets lost 3,050 men killed, 11,750 wounded and 250 missing, as well as 60 tanks, out of a strength of 60,000 men, 300 pieces of artillery and 150 tanks, while the Finns of Kenraalimajuri Armas-Eino Martola’s (from 6 July Kenraalimajuri Aarne Blick’s) 2^nd Divisioona, later reinforced by Kenraalimajuri Erst Ruben Magus’s Panssaridivisioona, lost 795 men killed, 4,975 wounded and 355 missing, as well as two assault guns, out of a strength of 30,000 men, 200 pieces of artillery and 35 assault guns.