In which war did America suffer the most casualties?

The American Civil War, fought from 1861 to 1865, resulted in the highest number of casualties in U.S. history, with estimates ranging from 620,000 to 750,000 soldiers losing their lives. This devastating conflict was primarily between the Northern states (the Union) and the Southern states (the Confederacy) over issues including slavery and states' rights. The war's impact on the nation was profound, shaping its future and leading to significant social and political changes.