The most obvious event in the lead up to the war on terror is, of course, the 9/11 attacks. However, I would submit that there were several other precipitous events before 9/11, such as the various embassy bombings and the destruction of the U.S.S. Cole in the late 1990's. That said, the U.S. has had to have major changes in the years following the 9/11 attacks. I'd say the most obvious change is in the realm of security. We've had the creation of the Department of Homeland Security, and entirely new cabinet post of the executive branch. We now have several safety procedures and checks that we never had before, for example airline security. I would also submit that the full effect has probably not yet been felt in American society... its important to remember that while the war is starting to be drawn to a close, it is still in fact going on, so it is hard to fully examine the changes as of yet.