Russia has become increasingly isolated on the global stage for over a year now as a result of its annexation of Crimea and role in the recent conflict in Ukraine. Nonetheless, it has been conducting military exercises over the last year, including in the Baltic Sea near the shores of Finland – as well as other northern European countries.
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Finland shares a land border with Russia but neither country is a member of the military alliance NATO, potentially making it more vulnerable to any future Russian aggression.
Seemingly aware of the potential military risks, the defense ministers of Nordic nations Sweden, Norway, Finland, Denmark and Iceland warned in April this year that Northern Europe must prepare for possible crises or incidents because of Russia.
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"Russia's leaders have shown that they are prepared to make practical and effective use of military means in order to reach their political goals, even when this involves violating principles of international law," the ministers wrote in a joint statement in Norwegian newspaper Aftenposten, Reuters reported.
Finland's army has around 16,000 members, although the number rises when including its Naval and Air Forces. The forces would be given a significant boost were reserves to be called up.
Reservists are those "who are liable for and have completed their military service," the Finnish Defence Forces (Finland's military) said on its website.
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The first letters were sent at the start of May with the final batch sent out in the last few days. Reservists were told by Finland's Defense Forces that within the letters:
"You will find your own details and your wartime assignment in the appendix with the letter. In other words, you can see whether you have an assigned wartime task, are in the reserve or have been reserved for other tasks by your employer."