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A buffer state is a country lying between two rival or potentially hostile greater powers, which by its existence is thought to prevent conflict between them. Buffer states, when authentically independent, typically pursue a neutralist foreign policy, which distinguishes them from satellite states. The conception of buffer states is part of the theory of balance of power that entered European strategic and diplomatic thinking in the 17th century. In the 19th century, the manipulation of buffer states like Afghanistan and the Central Asian emirates was an element in the diplomatic "Great Game" played out between the British and Russian Empire for control of the approaches to strategic mountain passes that led to British India.
Other examples of buffer states include:
- Canada[dubious ], during the Cold War, between the United States and the Union of Soviet Socialist Republics. In some scenarios, to reach the U.S., Soviet forces would have had to cross Canadian territory and fly through Canadian airspace. Russia viewed Canada more favourably and with less belligerence than the U.S. (particularly after the Avro Arrow and Bomarc missile affairs—which, in essence, meant that Canada was no longer, independently of the U.S., a military threat to the U.S.S.R. after 1972). A Soviet invasion of Canada would almost certainly have triggered war, and both the U.S. and Canada planned for such scenarios actively and intensively—not because the Soviets were thought to have independent, direct belligerence towards Canada, but because of the manifest necessity to use Canada, in a Soviet-U.S. post-Cold War wartime scenario. Legally, by international law between Canada and the U.S., an attack on the U.S. is viewed as an attack on Canada, and vice versa (because of NORAD) although in any reasonable geopolitical wartime strategy, Canada would be deemed distinct from the USA.
- Uruguay served as a demilitarised 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 American colonies that comprised the Atlantic Seaboard.
- Qasim Khanate, between Muscovy and Kazan Khanate.
- Tibet was a buffer between czarist Russia, the British Raj, and Qing China in the early 20th century.
- Mongolia, between the People's Republic of China and Russia.
- 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.
- The Sultanate of Aceh, located on the north part of Sumatra, as a buffer state between Kingdom of the Netherlands, ruler of Dutch East Indies and British Empire, ruler of Malaya.
- Siam — The king of Siam (now Thailand) 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 French Indochina and the British Raj.
- The Far Eastern Republic was a buffer state separating Bolshevik Russia from 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.
- The Himalayan nations of Nepal, Bhutan and Sikkim were buffer-states between the British and Chinese empires, later between China and India, which in 1962 fought the Sino-Indian War in places where the two regional powers bordered each other.
- Kingdom of Hungary, and later Transylvania between the Austrian Empire and Ottoman Empire; see also Banat.
- Poland following World War I, located between Germany and the Soviet Union.
- The Republic of Central Lithuania, existing from 1918 to 1922, was a buffer state between the Second Polish Republic and the Republic of Lithuania.
- Neutral Austria, Sweden and Finland were buffer states during the Cold War.
- 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 demilitarised buffer-zone between France and Germany during the inter-war years of the 1920s and early 1930s. There were early French attempts at creating a Rhineland Republic.
The invasion of a buffer state by one of the powers surrounding it will often result in war between the powers. For example, in 1914 the German invasion of Belgium triggered the entry of Britain into World War I.
The earlier forms of highly defended border regions, where defensive castles stood at a distance of a day's march are discussed at Marches. Some political remains of borderland marches established under the Carolingian and Ottonian Empires can be seen on the European map today: Belgium, Luxembourg, Lorraine. The Carolingian Empire also created some independent duchies in the Pyrenean border acting as buffer states against the Muslim kingdoms, an area called the Hispanic March, giving form to today's Andorra, Catalonia, Aragon and Navarre.