It was not till I read Sean McMeekin's excellent book Stalin's War that I realised that Russia invaded almost as many countries as Germany in 1938-40: Japan, Finland, Poland, Romania, Estonia, Latvia, Lithuania.

You should read Sean McMeekin's meticulously researched book Stalin's War. It has a lot of new information that came to light after the Soviet archives were briefly opened in the 90s. The book completely reoriented my view of World War 2 and the pivotal role Stalin played.

In the Asian theater, World War 2 arguably began in 1937 with the Japanese attack on China. One of the interesting things about McMeekin's book is that it unifies the European and Asian theaters from the Soviet perspective.