World War II or the Second World War (often shortened to WWII or WW2), was a major global conflict between 1939 and 1945, including almost the entire world except for South America and parts of Africa. The war was fought primarily between the Allies (British Empire, China, France, Soviet Union, United States of America etc.) and the Axis Powers (Germany, Japan, Italy, Hungary, Finland etc.). In total, 16,000,000 Allied soldiers and 8,000,000 Axis soldiers were killed throughout the war, along with 49,000,000 civilians in total. Swedish involvement in the war was limited, but Sweden did commit much trade with many major nations on both sides of the conflict until the United States ordered a trade embargo against Sweden, leaving Nazi Germany and Finland the only active trading partners.
See also: Swedish intervention in the Winter War
On 30 November, 1939, the Soviet Union invaded the southeastern borders of Finland. Although the Soviets were more than twice the Finnish numbers, the Finns relied on trained soldiers while the Soviet Army only sent poorly motivated conscripts into the war. It would take a few battles for the Soviet Union to realize that the war had to be taken seriously, and the "real" invasion began with 6,541 tanks and 3,880 aircraft (compare with Finland's 32 tanks and 114 aircraft). The Finnish government plead for foreign assistance, but only Sweden and Germany replied. The latter was already occupied with fighting the Soviets on their own border, and thus only the Swedes could actually provide useful aid to Finland. 10,000 Swedish soldiers, supported by 25 aircraft, crossed the border in early 1940 and could soon join the frontlines. The war ended, however, with the Finnish government surrendering despite having tactical superiority (much thanks to the Swedish assistance). 25,904 Finns and 28 Swedes were killed in the war, while the Soviets lost as many as 126,875 soldiers. It is disputed if the result would have been different if only the Finnish government would have kept on fighting, since the Soviet Union was unable to push further into Finland at the time and the Swedish support only increased. Some modern theories are that if Finland had not surrendered but went on the offensive together with Nazi Germany and Sweden, they would have been able to push an endless wave of troops into Leningrad, which if captured would give the Axis powers a clear route to Moscow.