Original sentence: The American Civil War was fought in the United States from 1861 until 1865 between the United States – forces coming mostly from the 23 northern states of the Union – and the newly-formed Confederate States of America, which consisted of 11 Southern states that had declared their secession
Edited sentence: The American Civil War was fought in the United States from 1861 until 1865 between the United States – of which 23 mostly northern states remained at the outset of the conflict (two new states would be admitted during it) – and the newly-formed Confederate States of America, which consisted of 11 seceding southern states