The Bush administration’s post-9/11 policies have done immeasurable harm to the U.S. and the world. Even a lot of the domestic political polarization and social crackup we see in America today can be traced back to the divisions that be Iraq War created.
The polarization started during the Civil Rights era of the 1960s. Or rather, the parties started sorting themselves on various issues after that point.
Ezra Klein has a good US-perspective/history in his recent book:
* https://en.wikipedia.org/wiki/Why_We%27re_Polarized
Humans were always polarized. The political parties in the US just didn't bother acting on that fact until the national Democrats turned their backs on the Dixiecrats, and the GOP then started their Southern Strategy.