Complex sentence: Co-belligerence is also the term used by Finland for her military co-operation with Germany in the Continuation War of 1941--44, when both countries had the Soviet Union as a common enemy.
Split into simpler sentences: Co-belligerence is also the term used by Finland for her military co-operation with Germany during the Second World War. Between 1941 and 1944, during what the Finns called the Continuation War, both countries had the Soviet Union as a common enemy.