War of the Triple Alliance

From Encyc

The War of the Triple Alliance was a war in which Brazil, Argentina, and Uruguay invaded Paraguay in the latter part of the 19th century. At the time, Paraguay was a significant regional power with more territory. When the war turned against them the Paraguayans refused to surrender, turning instead to more and more desperate guerilla tactics. This lead to a brutal invasion of their heartland and the death of the majority of the adult male population.