Two countries that are at war or hostile toward each other are sometimes separated by a buffer state, or a third, neutral country. Paraguay was a buffer state between Brazil and Argentina during the late 1800s.
Diplomacy in Europe after the 1800s relied heavily on buffer states to keep things calm between more powerful countries. Buffer states don't allow neighboring countries to have a military presence within their borders, and if one of the hostile states invades or attacks the buffer state, the result is usually a military conflict between powers. The buffer in buffer state is defined as a "barrier that reduces shock between antagonistic people or things."
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