Finnish forces facing the Karelian Isthmus consisted of two Finnish army corps. The Finnish II Corps was north of Vuoksi River and the Finnish IV Corps south of it. Finnish II Corps was commanded by Major-General Taavetti Laatikainen and it consisted of three divisions (10th, 15th and 18th divisions) as 10th Division had been added to it after the II Corps had been forced to give the 2nd Division over to operations in the Ladoga Karelia. Finnish IV Corps was commanded by Lieutenant General Karl Lennart Oesch and had two divisions and a reinforced regiment placed at front lines (8th and 12th divisions together with reinforced 25th Infantry Regiment) and single division (4th Division) as its reserve.
When the operation started the defending Soviet forces consisted nominally of 2 separate army corps (19th and 50th corps). Soviet 19th Corps consisted of two divisions (115th and 142nd divisions), one motorized division (198th Motorized Division) and a motorized regiment (14th Motorized Rifle Regiment). Soviet 198th Motorized Division was tied down in fighting near Sortavala while the two other divisions manned the positions close to the border. Soviet 265th Rifle Division was en route to act as reserve. The relatively quiet front prompted Soviet leadership to transfer the command elements of the 50th Corps to south of Leningrad on 21 July leaving its divisions (43rd and 123rd Rifle Divisions) under direct command of the Soviet 23rd Army.
The advance of the Finnish II Corps started on 31 July. Soviet defenses slowed down the Finnish advance especially due to the Finnish tactic of advancing through forest which caused severe logistical problems. By August 14 the Finnish 18th Division captured the town and crossing point of Antrea which left the Soviet 115th Rifle Divisions separated from the rest of the Soviet 19th Corps. Advance in almost terrain that lacked useful roads also slowed the Finnish 15th Division's advance and it only managed to capture the town of Hiitola on 11 August after the Finnish 10th Division was also deployed to the front lines. The Finnish victory at Hiitola forced the Soviet 142nd Rifle and 198th Motorized Divisions to withdraw into Kilpola islands where they were surrounded into a 'motti' against the shore of the lake Ladoga. Finns cleared the motti by 23 August but by then Soviets had already evacuated over the lake Ladoga 26,000 men from the encirclement.
The Soviet control of the Karelian Isthmus near lake Ladoga was crumbling after the defeat of the two Soviet divisions. Finnish 10th Division came across the newly arrived Soviet 265th Division on 15 August and after the ensuing battle encircled the remains of the Soviet division. A small part of the 265th Division managed to escape two days later; by that time division's casualties already amounted to 234 dead, 1155 wounded and 4830 missing in action. The Finnish victories allowed Finnish forces to act more freely and Finns captured the remains of the town of Käkisalmi on 21 August and village of Taipale on 23 August. Finnish 18th Division started its crossing of Vuoksi river on 17 August and managed to create a solid bridgehead.
The main objective for the Finnish IV Corps was the city of Viipuri and the plan called for surrounding and quickly capturing the town. However the Finnish General HQ didn't allow IV Corps to start actively pursuing the Soviets until August 21. By this time the Soviet 43rd and 123rd Rifle Divisions had already started withdrawing from their exposed positions close to the border while the Soviet 115th Rifle Division was racing to contain the Finnish crossing of Vuoksi. This meant that the Finnish plan of tying down the Soviets had failed before it even could have been put to action. However the crossing of Vuoksi by the Finnish 18th Division of the II Corps was assisted by the Finnish 12th Division and the Light Brigade T (named after its commander, Colonel Tiiainen, consisted of 1st Jäger Battalion, two light detachments and two artillery companies) of the IV Corps which managed to punch through the Soviet lines.