Finland was invaded twice by the Soviet Union during WW II. First, it was invaded during the Winter War (November 1939-March 1940). Finland had rejected several Soviet demands, including a demand for substantial territory in exchange for land elsewhere. The Soviets cited the need for security for Leningrad. After a Soviet victory, in which Finland put up a stubborn resistance, the Soviet annexation of Finnish territory was more severe than the original demands. The second invasion occurred during the Continuation War, (June 1941-September 1944) when Finland joined Nazi Germany in invading the Soviet Union. Finland defended its actions as a means to regain recently lost territory and the need to defend itself against its larger neighbor. Despite its defeat, Finland maintained its independence.

Latvia, Lithuania, and Estonia, collectively the Baltic States, like Finland, gained their independence from the Russian Empire in 1918 after WW I. The Baltic States, however, were in the precarious position of being in between Nazi Germany's and the Soviet Union's sphere of influence. In June 1940, Soviet troops had occupied the Baltic States which were officially incorporated in the Soviet state by August, losing their autonomy.

The Baltic States were then invaded in 1941 during the Nazi invasion of the Soviet Union, which reoccupied the region in 1945.

Resistance to the Soviet occupation continued and ultimately all three states regained independence on September 8, 1991.

More Info: en.wikipedia.org