The War of the Triple Alliance (1864-1870), pitting Argentina, Brazil and Uruguay against Paraguay, continues to provoke debate over its causes and implications. No antique quarrel between battlefield historians, the historiography of the War revolves around the basic questions of nineteenth century economic colonialism, imperialism and dependency. My contribution to the continuance of the discussions is that the nature and outcome of the war were shaped significantly by the participation of the Triple Alliance's indispensible fourth ally, Great Britain—in some ways, the most implacable of all independent Paraguay's nineteenth century foes.