Soviet Union was threatening Finland again in the fall of 1940 and Finland tried to seek political support from Germany, since Western allies were defeated and powerless at that time.
Germany wanted foreign Nordic nations to contribute men to new SS-formations. Finnish government saw this as the continuation of the 27th Royal Prussian Jäger Battalion, which had been a unit made of Finnish volunteers for the liberation of Finland from Russians. The unit fought in the Eastern Front in WWI and served as the basis for Finnish officer corps during Independence War of 1918. More important fact was that if a Finnish unit would be serving in the German Army, it would assure German political backing against the Soviet Union. The Finnish government would have preferred service in Wehrmacht, but Hitler wanted to use foreign volunteers in creation of his new national socialist army.
Recruiting was done in secrecy so that Russians wouldn't notice anything. A batch of about 1000 men was sent to Germany in spring of 1941 using German merchant shipping. Men arrived at Stralsund. Soon Operation Barbarossa begun and about 400 men, who had prior military training and combat experience were sent to SS-Division "Wiking" without training. They fought with "Wiking" from the Soviet border to Mius River in various battalions. Others, about 1000 men, were sent to training in Ostmark (Austria) in a town called Gross-Born. Unit trained there and it was named as 'Finnisches Freiwilligen-Batallion der Waffen-SS'. Training was complete in December 1941, and unit was sent to "Wiking" Division which was then part of Heeresgruppe Süd.
"Wiking" was on defense during spring of 1942 along Mius River. On 28th June 1942 began "Fall Blau", the German summer attack. "Wiking" went with Heeresgruppe A all the way to the Terek River in Chechenya. Fighting was heavy there and losses high. After German retreat from Caucasus in early 1943 battalion was held in reserve.
Finnish government had no illusions on the outcome of the war after Stalingrad, and battalion was at the end of the two year contract. New contract was refused, battalion came back to Finland on 1st June 1943 , and was disbanded. Men were sent to Finnish units. Finnish troops did not took part in any atrocities, though they witnessed some.