Initially the Finnish army was deployed in a defensive posture, but on June 29 Mannerheim created the Army of Karelia, commanded by Lt. Gen. Erik Heinrichs, and ordered it to prepare to attack Ladoga Karelia. The Army of Karelia consisted of VI Corps (5th and 11th Divisions), VII Corps (7th and 19th divisions) and Group Oinonen (also known as Group O, Cavalry Brigade, 1st Jaeger Brigade and 2nd Jaeger Brigade). Finnish 1st Division was kept in reserve. Finns planned to separate the defending the Soviet forces by reaching the shore of lake Ladoga and then advance along the shores of the lake.
Opposing them were the Soviet 7th Army with the 168th Rifle Division near Sortavala and 71st Rifle Division north of Jänisjärvi ("Hare Lake"). The Soviets had prepared field fortifications along the border across Sortavala and at the important road crossings at Värtsilä and Korpiselkä.
On July 9, the order for the offensive was given. The main task to break through the Soviet defences between Värtsilä and Korpiselkä was given to VI Corps, commanded by Maj. Gen. Paavo Talvela. The Finnish offensive quickly overwhelmed the Soviet defenders. Finnish 1st Jaeger Brigade (Col. Ruben Lagus) was brought from Group O to spearhead the assault and it managed to create a gap to the Soviet defences through which Finnish light infantry, some mounted on bicycles, pushed forward.
The right flank of the Finnish offensive consisting of the Finnish 11th Division of the VI Corps met strong Soviet resistance on the eastern shore of the Jänisjärvi Lake and clearing the resistance lasted until July 16. After clearing the resistance the 11th Division advanced and rounded the southern end of the Jänisjärvi Lake and set up positions facing west along the Jänisjoki River. Simultaneously the Finnish VII Corps had been attacking towards south on the western side of Jänisjärvi Lake however strong Soviet defensive effort turned the offensive into a crawl. It took until July 15 for the Finnish forces to reach the main Soviet defenses. It still took until July 17 for the Finnish VII Corps to finally reach the Jänisjoki River and clearing of the surrounded Soviet forces lasted until July 21. Since the Finnish advance had extended the frontlines some of the Finnish forces were starting to redeploy on July 16 with Finnish 1st Division being ordered to cover the eastern flank of the advance while the Finnish 17th Division which had left guarding of the Soviet base at Hanko to local troops was brought to the area as well. The two regiments strong German 163rd Infantry Division was ordered to capture the town and railroad junction of Suvilahti. These acts had effectively increased the Finnish strength in the area by three divisions.
The Finnish advance on the left flank of the VI Corps by two brigade strong Group Oinonen stalled almost as soon as it had started. While its advance tied down some Soviet troops, Talvela who commanded the Finnish VI Corps assessed that Group Oinonen's mission had been a resounding failure. However he also criticized his superior's orders to use these lighter troops against known strong Soviet positions.