There were substantial responses- not wholesale invasions, so they had little effect, and possibly made things worse.
In response to the bombings of the WTC and African Embassies, the US funded multiple assasination attempts against Osama bin Laden and his friends.
They also launched airstrikes and cruise missile attacks at him (including one on the night of Clinton's impeachment, which killed nothing but innocent civilians). The USS Cole attack was in self-defense, since it had been launching those missiles. The Sept.11 airplane attacks were probably invented as a way to duplicate the US's cruise-missiles on the cheap.