Andrew Johnson should have been convicted when he was impeached.

yup. for his original crime of colluding with the vietnamese to draw out the war so he'd win election.

LBJ even called it treason. https://www.bbc.com/news/magazine-21768668

Expounding upon the Civil War origins of our problems:

Every member of Confederate "high command", both military and civilian, should have been executed.

The wealth and property of any slaveholding plantationers should have been seized by force and redistributed to the formerly enslaved to follow through on the "forty acres and a mule" edict. Allowing slavers to keep and/or regain control of their wealth was a critical error.

The South should have had their participation in the federal government stripped away from them and been under US military rule for at least a generation.

Of course all this assumes that there was the public heart to actually stand up for black people and truly punish the slavers, where the reality was at best complicated, and more likely highly disappointing.

Were they indirectly responsible for the rise of the alt-right?

Maybe the US should've become a socialist country during the Great Depression.

And read and study Marx, Lenin, Trotsky and Bernstein?

Agree. People tend to not realize that the system itself is the problem, not the people currently at the top of the pyramid. They are exchangable, as long as the (capitalist) system itself is still in place

Maybe the US should've become a socialist country during the Great Depression.