Weapons increased in firepower. Weapons such as artillery and riffled/breach loading (rather than smooth-bore) rifles were prevalent. This made direct offensive operations more deadly and less effective. Whereas a direct infantry charge during the Napoleonic wars had to cover 50-60 yards under direct rifle fire - civil war rifles were effective at 500- 600 yards and could be fired 3x per minute.

Modern inventions likes railways, telegraphs, iron clad ships, torpedoes, aerial observation and photography were first used in this conflict.

The Overland campaign is usually considered the first modern campaign due to the continuous nature of the battles. Prior to this, most warfare was maneuver leading up to a decisive battle and then withdrawal of one of the armies.

Look at the dates of the "battles" in the Overland campaign.

The Wilderness (May 5–7, 1864) Spotsylvania Court House (May 8–21) Yellow Tavern (May 11) Meadow Bridge (May 12) North Anna (May 23–26) Wilson's Wharf (May 24) Across the Pamunkey (May 27–29) Haw's Shop (May 28) Totopotomoy Creek/Bethesda Church (May 28–30) Old Church/Matadequin Creek (May 30) Cold Harbor (May 31 – June 12) Crossing the James (June 12–18) Trevilian Station (June 11–12) Saint Mary's Church (June 24)

It was nearly 45 days of continuous fighting, leading to the siege of Petersburg.

This was really an extended engagement of maneuver using the extra divisions of the north to continually flank the south without any decisive battle taking place.

How did the Crimean War compare to the American Civil War in terms of being a "modern war"?