Have the American civil war wreck the American South even more so that it becomes a massive drain of the USA economy and have rebels pop up every so often attacking the goverment like the IRA.
The POD can not be CSA wins the war and gets annexed later.
For whatever reason, the South never surrenders, and the next few decades consist of guerrilla attacks and reprisals. Executions and incidents of such barbarity happen to the extent that neither side is willing to lay down their arms.