American Civil War vs United States Civil War

American Civil War and United States Civil War both are nouns.

Word	Noun	Adjective	Verb	Adverb
American Civil War	Yes	No	No	No
United States Civil War	Yes	No	No	No

As nouns, American Civil War and United States Civil War are synonyms defined as:

American Civil War and United States Civil War: civil war in the United States between the North and the South; 1861-1865

Other synonyms of American Civil War include War between the States.

American Civil War (noun)	United States Civil War (noun)
civil war in the United States between the North and the South; 1861-1865	civil war in the United States between the North and the South; 1861-1865