It is not about nationality, or ethnicity, or anything else about people living there, except international recognition of the territory and the amount of sovereignty it has.
It is about geopolitical situation, where states A and C happen to have state B between them. When relations between A and C go real sour, then B becomes a buffer state between them (unless B is powerful enough to kick asses of both A and C, should anything escalate). The buffer state’s role in geopolitics is to get invaded first on the way, cause an international outrage, and hopefully defuse the tensions before A gets to “C proper” territory, or at least stall the invasion.
> except international recognition of the territory and the amount of sovereignty it has
You can be 100% sovereign and be a buffer state and 0% sovereign and not be one (see French Polynesia)
> It is about geopolitical situation, where states A and C happen to have state B between them
then Germany is a buffer state, Poland is a buffer state, Israel is a buffer state, Afghanistan is a buffer state. Sweden is a buffer state, Finland is a buffer state, Bulgaria is a buffer state, Switzerland is a buffer state, Mongolia is a buffer state, Canada is a buffer state etc.
except that
Ukraine has been described as a buffer state between Russia and the NATO bloc, at least up to the ousting of former President Viktor Yanukovych in February 2014
We let them be an independent Nation when they agreed to become a kinda democratic country.
Donbass is the buffer state there, between Ukraine and Russia.
As far as we are concerned, Russia is not at war with Europe or NATO, but with Ukraine, for reasons belonging entirely to a Russian-Ukraine conflict.
A buffer state is a "neutral area" that two powers agree on, Russia never really agreed on Ukraine independence and tried to make it a buffer between them and NATO. If Russia wins the war against Ukraine, then Ukraine will become a buffer state.
If Ukraine was really a buffer state, we would be at war with Russia now, we surely would be if Russia attacked Finland or Sweden or Bulgaria or Romania or Poland, because it's Europe, or if USSR entered west Germany or when Iraq invaded Kuwait etc. etc.
What happened when Yugoslavia (a buffer state) dissolved?
Do you really believe USA would have missed a chance to go to war, given the motivation?
It is about geopolitical situation, where states A and C happen to have state B between them. When relations between A and C go real sour, then B becomes a buffer state between them (unless B is powerful enough to kick asses of both A and C, should anything escalate). The buffer state’s role in geopolitics is to get invaded first on the way, cause an international outrage, and hopefully defuse the tensions before A gets to “C proper” territory, or at least stall the invasion.