The thing is, and I ponder about this, we've seen Russia hit Georgia and Ukraine, we've seen their playbook so far, we know about the 'Little Green Men' and so forth, so having seen Moscow play those cards already, if sudden 'Little Green Men' started appearing in Estonia then surely alarm bells would start ringing in Brussels? Hence why NATOs current objective seems to be the preparation and practicing the deployment of rapid reaction forces. It's not something that's going to stop a major Russian advance, but it's not meant to, it's meant to slow it down until reinforcements can be deployed. Of course, if there's no political willpower to deploy those reinforcements then the whole thing is doomed from the start.
The Beeb did a fairly decent job of this a few months ago:
http://www.dailymotion.com/video/x3q8go9