How has water been the source of poltical conflict?

Water is a necessary resource on earth and every country wants it for its own people. A lot of arguments and conflicts started between countries just because of them wanting to use a water source that is shared between them. Example, if a river is going downstream between two countries and the country that has the upstream on its land builds a dam to save water. Then, the country that uses the downstream of this river would get upset and would argue over the water and who is supposed to get how much water from this river. 

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The study of where things are 

Ithink that the missing word is mixed