It seems weird to me that poor white farmers would go to war to preserve rich plantation ownership of slaves. What spin did the south put on the war to convince a poor white farmer to fight for the right to own slaves if he wasn’t rich enough to benefit from that right?
Listen to Episode 108 of the AskHistorians Podcast an interview of the historian Keri Leigh Merritt by /u/ThucydidesWasAwesome on this very topic.
The slave system itself served as propaganda. The poor yeoman farmers, sometimes destitute, but always laboring, could live with a certain amount of pride, that at least they weren't slaves, and they were a part of a grand agrarian lifestyle.
Land and slaves were what determined wealth in the Old South, and many whites (including the yeoman farmers and hired hands) possessed neither. Planters used their seats of power to control major social institutions such as churches and colleges, reinforcing their own self-serving narratives. In particular, the south's racial ideology which stressed the superiority of all whites to blacks. Thus, slavery became the basis of equality among whites, inflating the status of poor whites and blunting the conflict among the (white) classes. This Herrenvolk democracy was supposedly the best thing you could have as a yeoman farmer, as long as you were on the white side of the line. Ironically, however, the growing plantocracy within the south kept political and economic decisions away from the yeoman. More than simple racial division, there was a grander narrative at play which the planters exploited.
President Thomas Jefferson was one of the most prominent in romanticizing the agrarian lifestyle: the toiling in the fields and the mixing of labor with the earth- ironic for a man who never worked on a farm and detested manual labor. The planters coopted Jeffersons' narrative of a yeoman paradise, and Jefferson let them. The planters allowed themselves to believe they were the chosen people of God apart of a higher order than normal townsmen, merchants, or manufacturers. Another narrative that served the plantocracy at the expense and suffering of the yeoman, free blacks, slaves, and Native Americans.
For more information about the yeoman, how Jefferson allowed the plantation system to erode his agrarian paradise, and how the yeoman were so similarly treated to native Americans, read "Mr. Jefferson Lost Cause", by Roger G. Kennedy.