We were on this trajectory well before 9/11. This goes back to the globalization of the 70s and the Cold War propaganda we were fed in schools.
You take whole generations and teach them "The American Dream" which is conveniently life in the 50s, a temporary elevation of the average American lifestyle (compared to the Great Depression, company towns, and all that). You tell them we have a strong middle class because of capitalism and democracy, something communism will never afford them, even though our economic success has nothing to do with our political and economic principles and everything to do with the fact that we were the only developed country left standing after WWII with the infrastructure capable of supplying most of the rebuilding countries. Then as the rest of the world rebuilds, and more countries start to develop, we begin to lose out on unskilled jobs as that American way of life requires a much higher wage than the Chinese way of life.
So now steel mills are closing, factories are moving over seas, the cost for raw materials increases, but nobody wants to tell their voters that the temporality of the 50s and 60s has ended. So we begin the promise of "bringing back jobs", "reopening the factories", "making coal king again", etc. And a lot of people, particularly in unexceptional towns with nothing else to really offer other than unskilled labor, cling to those promises because what else do they have?
30-40 years of economic decline takes its toll on them. They get more and more desperate, they keep voting for the same things and get no change, every time being told it's someone else's fault. It's the Chinese sweat shops, it's the Democrat's high taxes, it's the EPA. And you've now created an "us" vs. "them", zero sum understanding of the economy.
9/11 helped to fracture the dividing line, but that fault was there long before. And the increased fervor we see today I consider less about patriotism and more about economic desperation. The commonality of the "true Americans" is that they have struggled with economic decline, all the while believing they're somehow the victims because they were promised a house, 2 cars, and a job that could support a family of 4 with a stay at home wife just with a high school diploma. And instead they're worse off this year than they were the last.
Trump came around and basically played the greatest hits. He'd bring back the jobs! He'd make coal king! It's all China's fault! It's all immigrants' fault! These people are being exploited by the government, by welfare queens, by everyone else who doesn't work as hard! And they bought into it so hard because they want to believe. And much like the Cold War, where we confused the collapse of the USSR as a solution to all our own problems, these people are thinking that making other people lose will somehow benefit them. And what's scary is it seems many of them realize that is no longer going to happen and now just want others to lose so they can take solace in not being the only losers.