There is only one person who is responsible for this war and that is Vladamir Putin. This was unprovoked. You don't send peacekeeping forces into a country that is already at peace. There also are no agreements anywhere that said that Nato would not add new members to the East after the fall of the Soviet Union. In the 90's there was even conversations about Russia joining Nato. Those are all Russian talking points and have been for decades.
I personally never thought it would get to this, but now I believe it is time to start taking Putin at this word. In his speech he gave earlier in the week he spoke specifically about reclaiming the Soviet empire. Given the events of today, the world has to believe that this is a start not an end. This is all going to escalate quickly, and I hope it can be contained before it reaches a cataclysmic level.
On a side note, I think it will be very important to see how Hungary responds to this. They are Nato members, but they are also currently being ruled by an Autocrat with his own visions of grandeur. He may start making overtures about rescuing the Slavs in Romania and we see a new war front open up.