* trench warfare, and "charges" into enemy lines so disastrously repeated in WW1 -- the lessons of the power of repeating rifles had not yet been learned.
* the first engagement between two "ironclad" ships https://en.wikipedia.org/wiki/USS_Merrimack_(1855)
* an early war that used railroads and telegraphs
* the first war that was photographed and had photographic journalism
* rifling + minie ball -- the US civil war was the first war using rifled muskets with special grooved bullets that could kill a man a half-mile away (800 meters), often shattering bones. It was the main reason why there were so many amputations.
* The gatling gun (predecessor to the maxim gun) was the first machine gun capable of firing a continuous series of rounds without reloading (the innovation was the creation of a belt of bullets that could be fed into the gun).
* medical innovations such as anesthetic inhalers, recognizing how to deal with chest wounds, life saving amputations, military ambulances, facial reconstruction surgery were invented in the US civil war (and contributed to much higher survival rates than in the Crimean War)
* Aerial reconnaissance (via balloons)
* naval torpedoes and naval mines were invented in the US civil war