A preventative war. With now Serbia able to do what it wants. But they broke the law and started doing some nasty things back there.
So defensive, yes, we don't want trouble in our region. Now Serbia is play along peacefully and they are prospering.
No NATO troops in Serbia.
You're arguing paranoid potential unlikely scenarios. Given the track record, the Russians are the aggressive ones and not Belgians, Czechs, etc. Seriously?
Russia is a huge country with poor infrastructure. It has a huge army. It has nukes. It would be suicide for anybody to attack them.
All you are afraid of is Russia not being able to bully its neighbours.