A buffer state is a country geographically lying between two rival or potentially hostile great powers. Its existence can sometimes be thought to prevent conflict between them. A buffer state is sometimes a mutually agreed upon area lying between two greater powers, which is demilitarized in the sense of not hosting the military of either power (though it will usually have its own military forces). The invasion of a buffer state by one of the powers surrounding it will often result in war between the powers.
Research shows that buffer states are significantly more likely to be conquered and occupied than are nonbuffer states. This is because "states that great powers have an interest in preserving—buffer states—are in fact in a high-risk group for death. Regional or great powers surrounding buffer states face a strategic imperative to take over buffer states: if these powers fail to act against the buffer, they fear that their opponent will take it over instead. By contrast, these concerns do not apply to nonbuffer states, where powers face no competition for influence or control."
Buffer states, when authentically independent, typically pursue a neutralist foreign policy, which distinguishes them from satellite states. The concept of buffer states is part of a theory of the balance of power that entered European strategic and diplomatic thinking in the 18th century.
Chapter 25: Destruction of the Buffer States between Germany and the Soviet Union.
Moscow's German Problem before Detente - The Federal Republic - In 1945, the major Soviet preoccupation was to prevent any future German attack; hence the imposition of Soviet-controlled governments in a ring of buffer states between Germany and the USSR.
Washington has a deep-seated interest in ending this conflict and maintaining Ukraine as a sovereign buffer state between Russia and NATO.
Instead of rushing to back the demonstrators who ousted the former president, Viktor Yanukovych, the United States and its European allies should have worked cooperatively with Moscow to craft a deal that would have preserved Ukraine's status as an independent but neutral buffer state.