Blaming protectionism is silly quite honestly.  Trade isn't nearly a large enough piece of the economic pie to cause a collapse of that magnitude.  GDP contracted by one-third, but trade was only about -- what, 15% of GDP?  And, really, tariffs simply aren't going to produce that kind of crash.

The causes can't be narrowed down to only one thing or another.  Was it the nature of incomes during the Gilded Age?  Was it contraction in the money supply?  Was it the logical result of an enormous credit bubble?

All of the above.

I don't think I'm opening any new doors to suggest that skyrocketing inequality and an increasingly crunched middle class should have a destabilizing effect on aggregate demand.  I think Friedman and Bernanke were both right to talk a lot about monetary policy accelerating the crash in output and employment because of further destabilization of demand through a combo of sticky prices and contraction.

Might the trade issue have damaged things?  I guess so, but, again, I think it's silly to suggest that tariffs caused it.

Conservatives want live babies so they can raise them to be dead soldiers. - George Carlin