There can't be anything in his speech about what the South was "fighting for" because at the time there was no war.
And wthen Stephens made his ""cornerstone speech"...it specifically states the 'cause' of the south was fighting for slavery!" there had already been years of a war of words in Congress between the Slave
States and Free States over Personal Liberty Laws, and a war of words about abolitionist and their demand of the end of slavery (a very long running war of words).
war of words noun phrase
Definition of war of words: an argument in which people or groups criticize and disagree with each other publicly and repeatedly for usually a long time