Soviet Invasion of Finland beginning in early December of 1939. If anything, it gave the Soviets an idea of just how ill-prepared they were to fight a war especially a Winter War and just how formidable an adversary the Finns were. Subsequently, the Soviet Union suffered a devastating morale blow to their credibility and the Finns gained a healthy respect from the rest of the world. The conflict finally ended in March of 1940 when a treaty was signed.
Here is an early report from December 9th, as broadcast by Teresa Bonney, who was a freelance photo-journalist working in Finland at the time. She gives details of the invasion and the evacuation of U.S. citizens from Finland in wake of the fighting.
An interesting report on a conflict somewhat overlooked during the early stages of World War 2.