Why has the U. S. government become increasingly concerned about terrorism?

The last thirty years has shown the globe the power of non-state actors using "terrorism" as a means to achieve political or social goals. For the United States, the bombings of the embassy in Beirut, the bombing of the USS Cole, and 9/11 has driven home the changing nature of warfare and the need to examine the root causes of terrorism, although the US has yet to achieve success in this arena.