The Military Districts of Perä-Pohjola and Pohjois-Pohjanmaa called up the 3rd and 6th Divisions, which concentrated in the area between Kajaani and Savukoski under the command of the peacetime V Corps.

As arranged in the previous month's discussions, the corps, led by Major General Hjalmar Siilasvuo, was formally subordinated to the headquarters of the German Army of Norway on 15 June.

[11] The German plans called for the III Corps, now consisting only of the 3rd Division, to cover the right (southern) flank of the Army of Norway.

Group F, consisting of the main body of the 3rd Division, attacked east from Suomussalmi; its initial objective was Voknavolok, from where it was expected to continue towards Ukhta.

As Siilasvuo did not trust the German officers to lead Finnish forces following their failures in the Salla region, the bulk of the combat strength of SS Division Nord was subordinated to Group J, the Finnish force consisting of a single regiment while the staff of the Division was subordinated into the III Corps HQ.

[19] Group J had taken up positions 8 miles (13 km) east of Kestenga after being pushed back by the Soviet 88th Division which had recently arrived in the area.

[19] General Siilasvuo reported to Generaloberst Nikolaus von Falkenhorst that III Corps was unable to reach Loukhi, with both sides settling for stationary warfare in the sector.

[21] By 6 October the situation at the front had improved to a point where von Falkenhorst and Siilasvuo discussed continuing the III Corps's advance towards Loukhi.

[22][23] To this end, on 16 October, von Falkenhorst approved a plan to capture "a crossing of a railway and a highway 30 kilometres (19 mi) east of Kestenga",[23] indicating the real purpose of the operation was to cut the Murmansk railroad.

On the same date, the headquarters of the German Army of Norway received a message from OKW, demanding an explanation for the III Corps's attack.

As part of these preparations, the Kannas Group, responsible for the Karelian Isthmus (Finnish: Karjalankannas), was split into two corps-level formations on 4 March.

[31] The 3rd Division, previously part of the III Corps and recently freed from its frontal duties in the Ukhta area, was subordinated directly to the commander-in-chief as reserve.

[35] The situation on the western Karelian Isthmus continued to deteriorate, and on 16 June Mannerheim ordered the 18th Division to be moved to the Vyborg region to act as his reserve as soon as possible.

[38] The front stabilized along the Vuoksi for the rest of the war, and Soviet forces failed to break the VKT-line on the III Corps sector despite several attempts.

However, following the failure of Operation Tanne Ost, where the German's attempted capture Gogland from Finnish defenders on 14–15 September, as well as a general cessation of any voluntary movement towards Norway, Finno-German cooperation completely broke down.

[41][42] During the resulting Lapland War, III Corps was moved to northern Finland where it took overall command of all the Finnish forces participating in the fighting against the Germans.

[43][3] As the Finnish Army completed its demobilization by early December, as mandated by the Moscow Armistice, the forces under III Corps were reduced from a peak strength of 75,000 to approximately 12,000 men.