The Civil War has accurately been described as the first modern war for a variety of reasons revolving around technology.
Transcontinental communications first came into play during the Civil War. Telegraph lines stretching out to California were pivotal in keeping Nevada and California firmly in the Union’s camp, while on the east coast dispatched from the front directed the movement of generals and the headlines in national papers.
The Civil War was also the advent of photojournalism, where photographers played a huge role in the documenting of the war. Photos taken on the battlefield, sometimes during the aftermath, swayed public opinion for and against the war based on the carnage showcased.
Naval technology improved significantly. The first true diving submarine was constructed in the south to try and break up the Union blockade. Ironclad vessels powered by steam made their debute in both the northern and southern navies, and by the end of the war steam powered ship were the norm.
The war saw the advent of repeating rifles, gatling guns, torpedoes, hand grenades and a half dozen other inventions on the battlefield. The patent office in D.C. issued thousands of patents, so many that the director of the office was convinced that his office should be closed because there was nothing left to invent!
Railroads were key in maneuvering armies and supplies around the battlefields, a system that gave the Union a critical edge.
However, it was the way that the war was fought which made it truly modern. Casualties were higher than they had ever been before, and the death tolls were, in some cases, the highest the U.S. would ever seen. The idea of Total War came into its own during the Civil War, in addition to the first example of trench warfare. Civilian populations were mobilized in a way they had never before been, with entire populations supporting the war effort. They also became targets in some ways.