Per the Oxford English Dictionary, reaffirm means:
- verb State again strongly.
‘the prime minister reaffirmed his commitment to the agreement’
1.1 Confirm the validity of (something previously established)
To affirm something is to state or demonstrate a point or a fact as definitely true. "The mutual defence treaty affirms the bond of friendship between these countries". "Marriage vows affirm the couple's commitment to one another".
To reaffirm something is to repeat a point or fact that has already been affirmed, in order to show ongoing commitment to it. "This new trade treaty reaffirms the bond of friendship between these countries". "Having children together reaffirms the couple's commitment to one another".
The idea is that since the American Civil War was fought over the rights of individuals, it is being seen as similar to the original American Revolution. Once again, American settlers fought to establish and maintain individual rights and freedoms, and to end a perceived injustice.