I've grew up near Russia, and I can describe you their mindset: all "agreements", "sanctions", "borders" etc don't matter. They are just distractions to prepare positions and advance their military. All that matters is force.
There's a Russian proverb: "All territories are ours, it's just someone temporary occupied them".
They also think in terms of territories (not people or economy) and victories. If they start losing a war, they start questioning their government.
So Estonia is doubly right to be worried, as, even if they actually manage to invade Ukraine, I see no reason they are going to stop at Ukrainian border. So the most logical step for Estonia is to help Ukraine as much as possible.
There is a significant Russian minority in Estonia (as in other Baltics) who after de-russification remain "second class" citizens (without the right to vote for example).
That is Russian propaganda, Estonian citizens all have the right to vote. There is a minority of Russians here who do not want to learn the language and become citizens, hence they can't vote. They have the attitude that we (Estonians) are occupying their land... it's crazy. Younger generation of Russians who are born here are mostly not like that though
*edit
Even non-citizen Russians have the right to vote in local elections, just not in nation wide elections.
Estonia is NATO since 2004. If Russia attacks Estonia ICBMs are launched and all wars are over for a long time.
I expect that in the next days some member of the US government, maybe the President, will publicly remember that Mutual Assured Destruction is what kept the world mostly at peace since the 50s, except small scale wars by proxy or with only one of the USA and SSSR directly involved.
So Russia won't invade, ICBMs won't fly and Ukraine becomes the next Afghanistan for Russia. Fast forward 20 years, Putin is in Lenin's Mausoleum and the new Iron Curtain could start to be lifted.
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?
This is constantly questioned by Americans, including the former president. It commonly goes like this: "I like $balticCountry, but are we really going to send nukes if $balticCountry is invaded?"
It might go down like WW2 when countries reacted only when large countries were invaded (Poland).
MAD works because the two parties do their best to convince the other party that they'd launch the missiles as soon as any of their allies or controlled territories are touched. If there is a crack in their resolve the other party can take a bite, then another one and then the alliance crumbles. Or they launch their missiles hoping for no retaliation.
Yes. Americans cannot find Estonia on the map, why starting World War 3 for it? Especially if it's not really clear there was an invasion, maybe there are real citizens that have some demands from their government, or some other shady things not clearly seen from afar?
There's a Russian proverb: "All territories are ours, it's just someone temporary occupied them".
They also think in terms of territories (not people or economy) and victories. If they start losing a war, they start questioning their government.
So Estonia is doubly right to be worried, as, even if they actually manage to invade Ukraine, I see no reason they are going to stop at Ukrainian border. So the most logical step for Estonia is to help Ukraine as much as possible.