 The Soviet government engaged in significant cooperation with Nazi Germany between 1939 and 1941, through extensive trade agreements which supplied Germany with vital raw materials for her war effort against Britain and France.As the other European powers were busy fighting in World War II, the USSR expanded her own military, and occupied the Hertza region as a result of the Winter War, annexed the Baltic states and annexed Bessarabia and Northern Bukovina from Romania.On 22 June 1941, Nazi Germany broke their non-aggression treaty with their erstwhile partner and invaded the Soviet Union with the largest and most powerful invasion force in human history, opening the largest theater of World War II.