One of the biggest factors that led to Soviet victory in WWII was the strength of its industry and production capability. One of the turning points in the war on the European theater was the Battle of Stalingrad. At Stalingrad, Hitler's intention to take the city was equally matched by Stalin's determination to hold it. While the Soviets lost more men in this one battle than the entirety of American losses for WWII, Stalin's defense prevailed and Hitler's forces were unable to take the city. Arguably, Stalin's victory was due in part to the strong industry and production. While Hitler's resources were dwindling and the supply lines inadequate, the Soviet Union found itself well-equipped for war, helping to propel the Soviets into an offensive position.
Another reason that enabled the Soviet victory was its location next to both Germany and Japan. As an ally of the United States and Great Britain, Russia agreed to the term of unconditional surrender. By May 1945, Hitler's reign had ended, Berlin had fallen to the allies, and the Russians occupied Germany. The next step was to secure unconditional surrender from the Japanese. In order to expedite the process, Russia declared war on Japan on August 8, 1945. This quote, from Foreign Commissar Molotov's announcement, details this event:
"The demand of the three powers, the United States, Great Britain and China, on July 26 for the unconditional surrender of the Japanese armed forces was rejected by Japan...Taking into consideration the refusal of Japan to capitulate, the Allies submitted to the Soviet Government a proposal to join the war against Japanese aggression and thus shorten the duration of the war, reduce the number of victims and facilitate the speedy restoration of universal peace..."In view of the above, the Soviet Government declares that from tomorrow, that is from Aug. 9, the Soviet Government will consider itself to be at war with Japan."
To wrap it up, Russia maintained a victory with its allies during WWII because of its strong industry and production as well as its proximity to the axis powers. The Soviet Union's large population, well-equipped army, and ability to maintain defenses and engage in offensive maneuvers were contributing reasons for Soviet victory in WWII.