The 1939 – 1940 Soviet invasion of Finland was one the least well known, but also among the most interesting of WWII campaigns. Here’s a site on the Winter War. It was also, by the way, the subject of a fantastic Finnish movie: THE WINTER WAR (Talvisota)
finland
Armies of Finland 1783 – 1811
The New York Public Library has a large collection of color plates of the armies of Finland in the 18th and early 19th centuries.