Reasons Why Russia Join The WWII?

WHY THE RUSSIA WANTED TO JOIN THE WWII
Do you know that the WWII? It is one of the biggest war in the world. In this war, there are many different kinds of aspects like economies , industry ,agriculture and so on inside it. So there will be many problems about the cruel war. My question is that why the Russia joined the WWII. In this war, what happen about the Russia? Someone think that Russia should not join the WWII. If they did not join the WWII , maybe they will not have so much injuries and deaths. Someone thinks that they must joined it, because if they did not join it, maybe there will be more injuries and deaths. I agree with the second part of people.
There are three reasons that I think Russia should join the WWII: First reason is
They loved their countries very much. So if they wanted to get the peace and welfare, they must prevent the German encroach on the eastern front. So Russia negotiated with General and Russia hoped if they helped German to fought the Poland with German together and divided the Poland, the German would not defeat them. They were too naive to believe that German will obey the contract. The primary resource is that “'We Forgot All Jews and Poles': German Women and the 'Ethnic Struggle' in Nazi-Occupied Poland”. This resource tell us that the German forget all of the Jews and Poles and Nazi Occupied Poland. The secondary reason is that “Poland's territory was divided between Germany and theSoviet Union, with Lithuania and Slovakia also receiving small shares..”(Spring, D. W. (1986). "The Soviet Decision for War against Finland, 30 November 1939". Soviet Studies 38 (2): 207–226.doi:10.1080/09668138608411636. JSTOR 151203.) . According to those source I know that German and Russia divided Poland. So Russia must joined the WWII. The another secondary resource is “When Light Pierced the Darkness: Christian Rescue of Jews in Nazi-Occupied Poland by Nechama Tec”. This resource tell us that this is a very dark times and Nazi occupied the
German was very unruly that teared the trade of the Molotov–Ribbentrop Pact and then Germany army killed a lot of soldiers of Russia. Germany were massing their forces for an attack suddenly. They used the awesome blitz to kill the Russia armies and destroy the Russia. They wanted to attack and occupy the Moscow. The primary reason is ‘ 1941: Germans invade Russia (Operation Barbarossa, June 22); Churchill and Roosevelt sign the Atlantic charter (Aug. 14).’ ("Global Information: History in Headlines: Modern Era." The 1998 Canadian Global Almanac. 1998. eLibrary. Web. 07 Apr. 2016.). This resource use the real fact to tell us that Germans invade Russia. The secondary reason is ‘ Winston Churchill and the Soviet Union during the Second World War. This resource comes from a university. It tell us the Germans attacked the Soviet Union. The another secondary source is WWII: Germany vs. Russia. In this resource, there is a video about the war that German attacked