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So right in the aftermath of the Uruguayan War of 1864-65 the powers of Brazil, Argentina, and somewhat Uruguay marched off to challenge the aggressive actions of the Paraguayan dictator Francisco Solano Lopez. The war lasted from 1864-1870 and was incredibly brutal in terms of lives lost and money spent, and arguably drained all the powers of South America.
Your challenge is to avert this war and speculate on the most likely outcomes of a South America absent such a war.
However, you cannot do this by preemptively killing Lopez