The invention that significantly advanced warfare during the Civil War was the rifled musket, particularly the Springfield Model 1861. It introduced a spiral groove in the barrel, allowing for greater range and accuracy compared to smoothbore muskets used previously. This advancement led to increased casualty rates, as soldiers could engage effectively from much farther away. Additionally, the widespread use of minie balls, conical bullets that expanded upon firing, further heightened lethality. The combination of these innovations marked a profound shift in military tactics and highlighted the brutal realities of modern warfare during the conflict.