Wikipedia states: "As a fascist government, the Kingdom of Italy had staunchly supported Francisco Franco during the Spanish Civil War in his fight against the Second Spanish Republic supported by the Soviet Union. Italy therefore promptly responded to requests by the Republic of Finland for military assistance and equipment for use against the communist government of the Soviet Union. The Royal Italian Air Force (Regia Aeronautica Italiana) sent thirty-five Fiat G.50 fighters, while the Royal Italian Army (Regio Esercito Italiano) supplied 94,500 new M1938 7.35 mm rifles for use by Finnish infantry. However, Germany intercepted most of Italy's aid and only released it once peace had been made.[4] A handful of Italian volunteers also fought in the Winter War on the side of Finland."
New answers welcome, but /u/wotan_weevil covered this previously. Short answer: the Molotov-Ribbentrop Pact.