Do Americans know the history of Arlington? I was quite amazed to learn how it became a military cemetery. Not hanging Robert Lee and the other high-ranking traitors is perhaps the biggest single mistake in US history. He is still the highest ranking US soldier to ever betray his country, no?
I think Benedict Arnold was a higher rank, but I guess it depends on if the US counted as a "country" at that time. Lee was a colonel when he betrayed the US and became a general under the Confederacy, while Arnold was a general at the time he betrayed the US.
Reconstruction did funny stuff and the mindsets at the time were different. The entire idea of the civil war centered around if we had a federal or confederated government. The confederate argument was states had to ratify the constitution, so therefore they could deratify and break away. Looking back this is obviously wrong, but a large minority of people believed this at the time, including the 15th President I believe. On the other hand, since Lee surrendered unconditionally, the thought process was you don’t kick the man while he’s down. Plus many men in the Union still considered Lee a heroic American figure. Not imprisoning him most likely led to an easier surrender by the rest of the Confederate forces and less paramilitary actions. Reconstruction further limited the punishment of top officials which was a mistake, though there was 0 chance they could end the thoughts and ideals that led to jim crow and the kkk, they could have certainly limited it.
I suppose it was obviously wrong in practice because the Union's greater population and industrial capacity made a Union victory almost inevitable, but I wouldn't say it was obviously wrong as a legal theory. The US was created by people who thought they had a moral right to leave the British empire and in that context it would be odd for them to intend that there should be no legal way for any state to leave the USA.
My understanding is that the US constitution was adopted illegally, anyway. The previous government didn't have a mechanism to do a full rewrite, I don't think. The adoption of the constitution via "fuck it, this isn't working" was almost in living memory at the time of the civil war.
I think the way US history is taught tends to be very civic religion-y