Who is NATO’s First Line of Defense against Russia?

It turns out that the newest NATO members, the ones that used to be part of the old Soviet Union are now NATO’s first line of defense if Russia decides to invade Europe.

Give the problems they are having finishing the war in Ukraine (they have won already, but the clean up is way slower than they thought) I’m doubting they would want to invade more countries.

But there are actually quite a lot of soldiers in these countries, from Estonia to Bulgara, all part of NATO and sworn to defend each other.