America had never been attacked on the continent until 9/11.  This act of terrorism instilled a fear in Americans never felt before.  Other than this, apparently little has been learned as the United States allows terrorists to come within its borders.

I think that one thing 9-11 taught Americans is that no one is truly immune from war.  I think we often have a tendency to become complacent in this country because wars are so often fought on someone else's soil.  It is hard for many American's to imagine what it would be like to live in a country where war was physically taking place.  Events like Pearl Harbor and 9-11 remind us that we are not the all powerful and completely protected nation that some believe us to be.

The idea of history ending due to the complete victory of liberal democratic capitalism is a very interesting one (brought up in posts 4 & 6). I'd agree that the attacks of 9/11 disproves the theory in the sense that economic organization does not wholly define the world's groups/groupings.

Doctrines and religions rather unrelated to nationhood and/or political states are capable of uniting populations in agreement and in opposition. 

I agree with Post 4. After the fall of communism, many people were seriously arguing that liberal democratic capitalism had vanquished all of its rivals, and that we had arrived at an "end of history," to paraphrase philosopher Francis Fukuyama. The attacks of 9/11 destroyed that notion in a matter of seconds, and it was only hours before we began hearing politicians talking about the democratic West's struggle against a new set of rivals. So yes, it was absolutely a reminder that history was not "over."

Obviously the biggest and saddest lesson out of 9/11 was that humans have the capability of planning and devising the death of thousands of fellow human beings, and that they do this moved by nothing but hatred. The biggest lesson is seeing with our own eyes how evil humans can be, how deeply they can hate and how crude can their outlook on life be. I have always said that what I saw on TV on 9/11 was the devil itself unleashing. It was horrible. That, and how this evil, horrible thing has produced a weaker and weaker government in the USA basically demonstrates that the deaths of those Americans have never been properly punished, nor can it ever be properly levered by anything. 

If the attacks taught us anything, they taught us that we are not done with history yet.  Of course, that should have been obvious, but we don't always see the obvious.  It taught us that even though we are getting so globalized and modern, there is still room for grievances and hatreds that can manifest themselves in terrible ways.  

From the standpoint of a citizen of the United States, I think that the events of 9/11 has shown us that our country is not invincible, that we are vulnerable to these sorts of terrorist attacks.  9/11 put a very human face on the tragedy of terrorism for many people in the U.S.

This is a great question. Let me make a few points that will get you started in thinking about this topic. 

The events of 9/11 teach us many things. First, it teaches us that for all our advancement and knowledge, some things never change or have a hard time changing. There will be conflict in our world and there is hate in the human heart. In other words, there is something about the human heart that is truly fallen and broken.  

9/11 also teaches us that religion is an important issue today. Many people thought that religion would die out by now with all our "enlightenment." However, there is a rise in religious beliefs all over the world. All statistics teach us that fundamentalism is growing in almost all major religions. This tells us that the human heart is religious.