One small country may have lost most of its men in a single war

From 1864 to 1870, Paraguay under Francisco Solano López fought Brazil, Argentina and Uruguay combined in the War of the Triple Alliance. Paraguay lost the war, lost territory, and lost an estimated 175,000 to 300,000 people — soldiers and civilians alike, out of a pre-war population of roughly 450,000 — mostly to battle, starvation and disease. It remains the deadliest interstate war in Latin American history relative to the population involved.

— Wikipedia contributors, Paraguayan War — War of the Triple Alliance, 1864–1870