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A buffer state is a country lying between two rival or potentially hostile greater powers, which by its sheer 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 Britain and Tsarist Russia for control of the approaches to strategic mountain passes that led to British India.

Other examples of buffer states include:

• Kingdom of Armenia between the Roman Empire and the Parthian Empire
• Qasim Khanate, between Muscovy and Kazan Khanate
• Kingdom of Hungary, and later Transylvania in the time of principality between the Austrian Empire and Ottoman Empire; see also Banat.
• Mongolia, between the People's Republic of China and Russia
• Nepal and Bhutan, between the People's Republic of China and India
• Poland following World War I, located between Germany and the Soviet Union
• North Korea during and after the Cold War, whom some analysts see as a buffer state between the military forces of the People's Republic of China and American forces in South Korea
• 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.
• Neutral Austria, Sweden and Finland were buffer states during the Cold War.
• Belgium before World War I, serving as a buffer between the United Kingdom, France, and Germany
• 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 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.
• Uruguay served as a demilitarised buffer-zone between Argentine Republic and the Empire of Brazil during the early independent period in South America.
• Paraguay which was maintained after the end of the War of the Triple Alliance in 1870 as a territory separating Argentina and Brazil.

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 Great 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 todays Andorra and the region of Catalonia.

Even earlier, compare the highly-defended Roman Empire's limes with its "client kingdoms" like Palmyra, Judaea, Numidia or Mauretania, and the Persian Empire's system of satrapies.

• satellite state
• puppet state
• buffer zone
• marches