Complex sentence: The Soviet fronts were first raised during the Russian Civil War and were activated for the Polish-Soviet War of 1920, the Soviet-Finnish War of 1939, the Invasion of Poland in 1939, and during the Second World War.
Split into simpler sentences: The Soviet fronts were first raised during the Russian Civil War. The occasions when one or more fronts were activated were: the Russian Civil War, the Polish-Soviet War of 1920, the Soviet-Finnish War of 1939, the Invasion of Poland in 1939, and the Second World War.