That's a very heavy accusation to lob at the US people I think. Who says this and where does the 80% number come from?
And why would the US people want their country to invade a country on the other side of the world, that was no threat to them and was clearly not capable of defending itself against the might of the US army? Why would the US people want their country to invade anyone? And 80% of them? This I find very hard to believe. Maybe the "80% support" was for something else entirely, e.g. killing Bin Laden or destroying Al Qaeda, rather than invading Afghanistan?
There are likely many factors. In terms of civilian supporting the invastion, or at the very least, being neutral on invading was very much a popular opinion. Patriotism was extremely high following the attacks, and whether it was correct or not, many folks would see not supporting the invasion as unpatriotic. So likely do to social norms and group dynamics, it was unpopular to be against the war.
It might be hard to understand for folks that didn't live in the US during that time. A time when TV and print media were very much king. To paint the picture more clearly. Imagine a 24-hour news cycle on nearly every single TV channel. Even non-news channels discontinued normal programing to broadcast one of the big news stations. Every newspaper has massive headlines covering the content. For days and weeks there was no escaping the media barrage. It definitely warped folks perception.
So a kind of mass indoctrination to a militaristic concept of "patriotism", as I understand it? A concept that is not satisfied unless someone is invaded.
And why would the US people want their country to invade a country on the other side of the world, that was no threat to them and was clearly not capable of defending itself against the might of the US army? Why would the US people want their country to invade anyone? And 80% of them? This I find very hard to believe. Maybe the "80% support" was for something else entirely, e.g. killing Bin Laden or destroying Al Qaeda, rather than invading Afghanistan?