This article is from the Nordic countries FAQ, by Antti Lahelma and Johan Olofsson, with numerous contributions by others.
As the political situation in Russia gets increasingly chaotic
after the revolutions in 1917, Finland prepares for liberation.
...or sooner: the Conservative farmers and the educated class
prepare for Independence. The agrarian and urban proletarians,
inspired by the October Revolution in Russia, instead prepare
for a World Revolution. Strikes, riots and shootouts occur in
several cities and towns; as well as some widely-publicized
murders.
The former organized so-called Security Corps - the latter Red
Militia.
As Finland's parliament declares Finland a sovereign state on
December 6th 1917, the "Security Corps" claim status as the
national army, and the polarity between the Corps and the Red
Militia aggravates further.
(The Ă…land Islands try to become independent too - from
Finland! - but fail to achieve this.)
According to a revoked law from 1878, a compulsory military
service is introduced, and the remaining Russian troops are
required to leave. As they don't, they are disarmed by the
National Army. This triggers the mobilization of the Red
Militias of southern Finland against the "White" government at
the end of January 1918.
The Civil War lasts only three months, but is both bitter and
bloody. Initially, southern Finland (with a majority of the
country's population and its major urban centers) is controlled
by the Red Militias, while the White government controls the
predominantly agrarian northern and central provinces.
Eventually, the White side defeats the Red, aided by
volunteering officers from Sweden (8,000 man) and Norway
(700 man), Finnish officers from the Czar's army, the Finnish
officers educated in Germany and additionally also military
support from Germany. Some 30,000 people (out of 3 mill.
population) die as a result of the war; when the Red fronts
collapse at end of April, the Militia leaders go underground or
flee to Russia; tens of thousands of rank-and-file surrendered
militia troops, male and female, are placed in prison camps.
Several thousands are executed. At end of May 1918, General
Mannerheim receives the White victory parade in Helsinki.
The Civil War is followed by enhanced orientation toward
Germany, and a German prince is proposed to become king of
Finland. As Germany loses the World War, this alternative
becomes politically unrealistic.
 
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