The Finnish military has begun sending letters to some 900,000 reservists amid increased tensions between the Nordic states and Russia over the Ukraine crisis.
Russia and Finland share a 1,340-kilometer land border.
The military said the letter campaign, which started on May 4, was prompted mainly by cuts in the defense budget.
Finland's defense budget this year has shrunk to 2.7 billion euros ($2.99 billion), or 1.29 percent of GDP, down from 1.4 percent in 2012.
The letters reportedly informed reservists what their role would be in a "crisis situation."
Finland, with a population of 5.2 million, is one of few countries in Europe still to have compulsory military service for men. It has a professional army of 16,000, but in the event of mobilization, it could muster a 230,000-strong force.
Last month, neutral Finland and Sweden joined with NATO members Norway, Denmark, and Iceland in a joint statement that directly cited the Russian "challenge" as grounds to boost defense cooperation.