Used it in from the end of WW1, through WW2 and onwards.
They fought with Nazi Germany during WW2, although they weren't in truth really 'allied' as such. More a case of fighting against the Soviet Union for their own self-determination.
I think, if memory serves me correctly, that Finland boasts the most successful sniper in the history of warfare 'The White Death'. Well worth reading up about. Over 500 confirmed kills.