With a POD no earlier than June 9, 1861, make one of the 11 OTL Confederate states (not preventing others from joining, just preventing their secession from the Confederacy) secede from the Confederacy and remain an independent republic. An entire 1859 state must secede from the Confederacy and not rejoin the Union or the Confederacy except through armed occupation, and the secession must happen during the American Civil War.