It’s the failure to adequately punish the actions immediately after the catastrophe. After WWII, every major Nazi the Allies could find was publicly executed. This included those who worked directly to advance the Holocaust like Goring to those who had no direct association with the Final Solution like Ribbentrop. Stalin knew that if Nazism wasn’t burned to the ground it would rise again. The rearrangement of Germany after the war furthered the dismantling of the forces that brought Nazis to power. That’s why Prussia, the most powerful German state for centuries, no longer exists. They had been the stronghold of political support for Nazism in the Wiemar Republic, and Churchill was steadfast on destroying that. After the Civil War, on the other hand, no Confederates were so much as given life in prison. The few restrictions on former Confederates were rolled back by the Amnesty Act of 1872. The Executive Branch largely worked to restore the antebellum structure rather than install a more just system. Why? Because after WWII, Germany was rebuilt by the people they’d attacked. After the Civil War, the South was rebuilt by a rich white man from North Carolina.
Furthermore, the resistance to military occupation after the war in Germany was minute compared to what occupying soldiers faced during Reconstruction (proportionately at least). While there were hold outs against the Allied forces, they never posed a significant threat. Military occupation ended because the mission was over and the threat had been suppressed (well… it was extended because of Cold War politics but that’s neither here nor there). Military occupation of the South was the exact opposite. Resistance was so strong that the occupation ended in 1877 after southern leaders called for a mob to sack the Capitol if they weren’t granted the Presidency (which they weren’t, but on the condition that Federal troops would be removed from the area). The Union victory in 1865 seemed decisive, but by 1877 it was clear that it was Pyrrhic.
In the popular Union marching song “John Brown’s Body” they chant “We’ll hang Jeff Davis from a sour apple tree!” and we fucking should have.
EDIT: I should also point out that there are Waffen-SS reenactors in Germany and Austria, but they’re very difficult to find as it is illegal.