The increased polarization in the United States is a product of the end in the mid-1990s of the overlapping pair of partisan realignments starting just before WWII and a return to the normal condition encouraged by the US electoral structure of a strong, stable, sharp partisan divide, combined with modern mass media technology.
The 2001 terror attacks are coincidental, not a cause.
Denying this is closing your eyes to major structural problems with the political system in the United States and, as long as you deny something, you are categorically unable to fix it.
We agree that this was already happening prior to 2001, but I simply doubt it would have resulted in the extreme situation that we have today. Social media, similarly is an accelerator, not a cause.
Denying this is closing your eyes to some major problems and as long as you deny something you are categorically unable to fix it.