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.
The machine gun changed warfare
How about this? "The Civil War was the birthplace of technologies that changed the nature of warfare forever."
sucking balls
ANSWER The building of the Ironclads.
There were a few better techniques due to technological advantage, one was trench warfare, gas warfare, the invention of machine guns, grenades, and artillery
Gunpowder
Iron Bridge
Aerial vehicles
gunpowder and the abacus.
World War 2 changed the world forever by teaching us a lesson in the areas of atomic warfare, and the treatment of war prisoners.
The machine gun changed warfare
The invention of the Firearm
zeus-- > Athena is a Goddess .
How about this? "The Civil War was the birthplace of technologies that changed the nature of warfare forever."
sucking balls
The invention of machine guns, gas warfare and trench warfare
If by "solution for trench warfare", you mean a way to end trench warfare, then the invention and widescale use of tanks, bombers and the Blitzkrieg in WWII solved trench warfare.