On April 14, 1865 which Lincoln, Johnson, and Seward are all killed simultaneously by John Wilkes Booth and his conspirators (as was their plan IOTL). The President Pro-Tempore of the Senate and Radical Republican Benjamin Wade takes over the government, and the South attempts to rise a second time only to be crushed. The second suppression of the South is a lot less patient than Lincoln's version of Reconstruction with citizenship and voting rights permanently stripped from anyone who served in the Confederate military or held any elected or appointed office of the Confederate government. Property is confiscated from any slave-owners who forfeited their citizenship in such a manner and awarded to freed slaves, loyal whites, and northern veterans (there was some corruption involved in this, unfortunately). State borders are redrawn in accordance with State Suicide Theory so as to maximize the number of states with majorities of African-Americans and Southern Unionists.
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It's funny. You don't even need Booth to kill all those people. The south actually nearly re-seceded in the 1880s over things like confederate banners, the rise of the First Klan and the attacks on "carpetbaggers, *African Americans, and scalawags" that made the south become even more insular and racist than before.