Why do many US historians consider the Civil War to be the first modern war in American history?

First war with extensive troop movement by rail.

First widely-photographed war.

First war where orders were sent by telegraph.

First war that used rifled cannon.

First war that used submarines, steamships and ironclad vessels.

NEW RESPONDENT.

I do think that following factors should be added to those mentioned above viz.

1 -The reorganization of the Union General Staff and the Grand Strategy adopted after Grant's appointment as General-in-Chief, whose effectiveness was judged to be equal if not superior of that of the Great Prussian General Staff during the War against Austria in 1866 and against France in 1860-71.

2 -The major tactic and logistic innovations in the field of employment of the infantry, artillery, cavalry and in the use field fortifications, adopted after the early stages of the war, which were not realized and implemented in Europe and throughout the world until the end of WW1.

3 - The adoption of the Total War Strategy.