A buffer state is a country lying between two rival or potentially hostile greater 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 in their stead. 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 17th century.
A march is a fortified non-homeland territory for defense against a rival power. A march is controlled by a greater power, whereas a true buffer state is deliberately left alone by rival powers situated either side of it.
Examples of buffer states include:
Uruguay served as a demilitarized buffer zone between Argentina and the Empire of Brazil during the early independence period in South America.
Paraguay was maintained after the end of the Paraguayan War in 1870, as a territory separating Argentina and Brazil.
The colony of Georgia in the 18th century, as a buffer state between Spanish-controlled Florida and the British-controlled American colonies that comprised the Atlantic Seaboard.
North Korea during and after the Cold War, seen by some analysts as a buffer state between the military forces of the People's Republic of China and American forces in South Korea and Japan.
Siam, whose king had to surrender his country's hegemony over Laos and Cambodia and to grant commercial concessions to France but managed to retain independence as a buffer state between British Raj, British Malaya, and the French Indochina.
The Far Eastern Republic was a formally independent state created to act as a buffer between Bolshevik Russia and Imperial Japan.
Afghanistan was a buffer state between the British Empire (which ruled much of South Asia) and Russian Empire (which ruled much of Central Asia) during the Anglo–Russian conflicts in Asia during the 19th century, with the Wakhan Corridor later extending the buffer eastwards to the Chinese border.
The Himalayan nations of Nepal, Bhutan and Sikkim were buffer-states between the British Empire and China, later between China and India, which in 1962 fought the Sino-Indian War in places where the two regional powers bordered each other.
Mongolia, acted as a buffer between the Soviet Union and China and currently serves as a buffer between Russia and China.
The Saadi Sultanate of Morocco served as a buffer state between the Ottoman Empire, Spain, and Portugal in the 16th century.
The Kingdom of Hungary, and later Transylvania between the Austrian Empire and Ottoman Empire.
Belgium before World War I, serving as a buffer between France, Prussia (after 1871 the German Empire), the United Kingdom and the Kingdom of the Netherlands.
The Rhineland served as a demilitarized buffer-zone between France and Germany during the inter-war years of the 1920s and early 1930s. There were early French attempts at creating the Rhineland Republic.
Qasim Khanate, between Muscovy and Kazan Khanate.
Poland and other states between Germany and the Soviet Union have sometimes been described as buffer states, with reference both to when they were non-communist states before World War II, and to when they were communist states after World War II.
Ukraine has been described by experts such as John Mearsheimer and Stephen Walt as a buffer state between Russia and the NATO bloc, at least up to the ousting of former President Viktor Yanukovich in February 2014.
