Complex sentence: Finland was declared an independent nation state and a sovereign republic on 6 December 1917, but after the Civil War, there was a brief attempt for own monarchy from 9 October to 14 December 1918.
Split into simpler sentences: Finland was declared an independent nation state on 6 December 1917. After the Civil War in 1918, there was a brief attempt to make Finland a kingdom from 9 October to 14 December 1918.