Short answers, pulled from the story.
Soviet forces crossed the border into Mongolia in August 1921. The Red Army moved from multiple directions to capture key locations while fighting against Roman Ungern Von Sternberg, who had conquered the country by February 1921.
Five youths launched a coup with Soviet support in January 1929 and deposed Donduk Kuular. They imprisoned him and later executed him after he tried to make Buddhism the state religion and resist Soviet collectivization policies.
Finland ceded 11 percent of its territory representing 30 percent of its economy to the Soviet Union. This agreement ended the conflict that began when the Soviet Union invaded Finland on the 30th of November 1939.
The Soviet Union occupied Estonia, Latvia, and Lithuania in June 1940. These states were incorporated into the Soviet Union as constituent republics in August 1940 before regaining independence in August 1991.
The Soviet Union invaded Poland from the east on the 17th of September 1939, sixteen days after Germany attacked from the west. Military operations ended on the 6th of October 1939 with the two-way division of the Second Polish Republic.