What if some small Estonian county says that they are majority Russians and want to do a referendum? Just like Scotland did (but didn't succeed). Just like Tran

What if some small Estonian county says that they are majority Russians and want to do a referendum? Just like Scotland did (but didn't succeed). Just like Transnistria did. Just like South Osetia did. Just like Crimea did. Just like Donetsk did in 2014. And then Russia wants to help them. Hey, our fellow comrades are under oppression from Estonian government! There can be many different recipes, but it's never a 1-day process. The current war with Russia is ongoing since 2014. First it's separatists (mostly really residents of those territories, supported by Russia), then it's some unknown bands of criminals. And very strong propaganda, that persuades everyone and their parliaments that those peoples' voices must be heard, etc. No ICBMs won't fly in that case, because it's always "not such a big deal for ICBMs". Are you going to start World War 3 if some small Estonian territory really wants to secede?