Gunpowder
Iron Bridge
Do you mean, what invention utilised during the Civil War changed combat? The rifle-barrelled cannon. Or the minie-ball. The post-war invention would have been the machine-gun.
Aerial vehicles
gunpowder and the abacus.
The invention of gunpowder in 9th century China significantly changed warfare. Initially developed for use in fireworks, it quickly transitioned to military applications, leading to the creation of explosive weapons, cannons, and firearms. This innovation revolutionized combat tactics and strategies, allowing for greater destructive power and altering the dynamics of battles both in China and eventually around the world. Gunpowder's impact extended beyond the battlefield, influencing the development of modern warfare.
Before the explosive devices were invented, fighting completely required strength (for example using swords, arrows etc.) and never relied much on heat to kill the enemy. The invention of the explosive devices changed this.
World War 2 changed the world forever by teaching us a lesson in the areas of atomic warfare, and the treatment of war prisoners.
Gunpowder - Tseng Kung-Liang made the formula publicly available in AD 1040.
Gunpowder: once used in warfare, it made it's way along the silk road to India, the Middle East and Europe.
The machine gun changed warfare
How about this? "The Civil War was the birthplace of technologies that changed the nature of warfare forever."
The invention of the Firearm