Buffer State
A country whose main purpose, in the eyes of its powerful neighbors, is to keep them from touching, and whose sovereignty lasts exactly as long as that suits both.
A buffer state is a country that exists, in the strategic imagination of its larger neighbors, mainly in order to separate them. Its borders were often drawn by outsiders, its neutrality is usually guaranteed by outsiders, and its independence survives as long as both sides prefer a gap to a shared frontier. The moment one of them stops preferring that, the buffer discovers what its guarantees are worth.
The logic is straightforward and old. Two great powers with a common border have a permanent crisis: every fort, road, and garrison one builds reads as preparation to the other. Insert a weak state between them and the friction drops. Neither can be surprised, both get warning time, and neither has to stare at the other's army.
The classic cases
Afghanistan is the textbook. Through the nineteenth century Britain and Russia converged across Central Asia in what the British called the Great Game, and rather than meet, they agreed to leave a mountainous kingdom in between. The Durand Line of 1893 fixed its eastern edge, and in 1895 the two empires deliberately drew the Wakhan Corridor, a strip of high valley barely a dozen miles wide in places, purely so that Russian and British territory would nowhere be adjacent. Afghanistan's shape is not geography. It is a diplomatic instrument.
Belgium is the European version. Carved out in 1830 and given guaranteed neutrality by the great powers in the 1839 Treaty of London, its function was to stop France and the German states, and to keep the Channel ports out of the hands of whoever was strongest. That guarantee held for seventy-five years and then became the legal reason Britain entered the First World War, when Germany treated it as a scrap of paper.
Others: Siam, alone among Southeast Asian states in staying independent, precisely because Britain in Burma and France in Indochina both preferred a cushion. Poland, which spent the eighteenth century as everyone's buffer before being partitioned out of existence. Nepal and Bhutan between India and China. Korea between China, Russia, and Japan, and dismembered when that arrangement broke.
The catch
Buffer status is not a favor. It comes with three costs.
Sovereignty is conditional. Buffer states rarely get to run their own foreign policy, because the whole arrangement depends on their not choosing a side. Finland spent the Cold War with a functioning democracy and no ability to join anything, which is why the word Finlandization exists.
The guarantee is only as good as the guarantor's mood. Belgium in 1914 and Afghanistan repeatedly discovered that a promise of neutrality is a promise about someone else's future intentions.
And when the buffer fails, it fails on the buffer's territory. That is the point of it. The wars that would have been fought along a shared border get fought inside the cushion instead. Afghanistan has been invaded by Britain three times, the Soviet Union once, and the United States once, which is a lot of invasions for a country nobody wanted.
The echo
The concept is not historical. The argument over Ukraine since 2014 is at bottom an argument about whether a buffer is what Ukraine is. Moscow's stated grievance is that a cushion it considers essential was being absorbed into the other side's alliance system. Kyiv's answer is that a nation of forty million people is not furniture, and that the security a buffer supplies to its neighbors is purchased with the buffer's own insecurity.
Both positions are internally consistent, which is why the dispute is so hard. The lesson from Afghanistan, Belgium, Poland, and Korea is narrower and grimmer: countries assigned this role by geography do not usually get to resign from it, and the safest outcome available to them is that both neighbors stay nervous forever.