The fact that Ukraine is not a NATO state is the only reason why Putin is maneuvering as he wants. Why do you think he's not touching Latvia or Estonia. Both have a strong pro-Russian population but both are NATO members. Putin can't do anything there.

Getting back to Urkaine, no one and I mean no one will intervene through military actions except Ukraine itself and they are no match for the Russians.
The question is where will it stop? Will Putin completely take over Ukraine (I doubt) or will he only focus on southern and southeastern towns along the border.
Novoazovs'k just fell down to Russia. It is a key town as it will now serves as an entry point for Russian soldiers through the southeastern borders.

How far can the US and EU push economic sanctions down Russia's throat before Putin shuts down oil & gas exports to Europe?