As 21st century activists seek to topple monuments to the 19th century Confederate rebellion, some white Southerners are again advocating for what the Confederates tried and failed to do: secede from the Union.It's not an easy argument to win, and it's not clear how much support the idea has: The leading Southern nationalist group, the Alabama-based League of the South, has been making the same claim for more than two decades and still has an address in the USA, not the CSA.…