This seems to be a question people have just accepted in the majority of the discussion here.
There are the obvious actions i.e. the war on terror and it's consequences. which would never have been possible without 911.
But why would the government wish to attack Afghanistan and Iraq, or anyone else for that matter without being provoked?
An obvious reasoning is oil. and almost certainly in Iraq is a big incentive.
A second reasoning is a little more complex. and is described in a "hoax" published in 1967 called The Iron Mountain Report, it was claimed to be funded by the american government, for more info read introduction from link below. Note: The report is completely denied as a hoax by the american government.
http://www.mega.nu/ampp/ironmtn.html
This does take a while to read, i suggest you browse the intro and conclusions to begin with.
Conclusions such as:
"determination of minimum and optimum levels of destruction of life, property, and natural resources"
Make for an extremely disturbing read, especially after 911. also bare in mind when this was published, and the similarities between the report and the political and social state of the world now.
there was also a good video on google videos that covered the topic, I'm not sure if its still there.