Complex sentence: When Finland declared independence on 6 December 1917, the "Avance" was still under Russian control, and after a failed attempt to recapture her in early 1918 she was taken to St. Petersburg by the retreating Russian revolutionaries.
Split into simpler sentences: When Finland declared independence on 6 December 1917, the "Avance" was still under Russian control. After a failed attempt to recapture her in early 1918 she was taken to St. Petersburg by the retreating Russian revolutionaries.