Estonia has a problem. Something like 20+ percent of the population is ethnic Russian. Near 10% are Russian citizens, not Estonian. When Estonia was a vassal state of the Soviet Union, the latter did as much as it could to Russify all three Baltic states. Estonia got it the worst. So as a NATO member, Estonia has significant exposure to internal subversion in time of war. It could be worse, the percentage was higher in 1991 when Estonia regained independence and thereafter many ethnic Russians chose to leave.