The Nazi party was led by Adolph Hitler who played a major role in demolishing the democratic parties of Germany. The other powers that helped Germany in this war were Italy, Japan and Bulgaria. On September 1st 1939 Germany attacked Poland where France and United Kingdom declared war against Germany. United States became a part of this war after Japan attacked pearl harbour which took place on 7th December 1941.
Why are you all saying that it started in 1939? It started in 1931 when the second Sino-Japanese (China vs. Japan) War happened. Then they invaded other countries and had Manchuria. However it ended in August 1945 when Japan surrendered (Germany already did).