On the one hand, there is no question that the United States is a much “harder” target for transnational terrorism than it was before 9/11. Likewise, federal, state, and local law enforcement agencies have paid much more attention to the threat of “homegrown” terrorism. Since the September 11 attacks, government agencies have thwarted over 19 conspiracies aimed at killing Americans on U.S. soil.
Nevertheless, it is unrealistic to believe that all homeland security efforts will deny every attack every time. In particular, armed assaults and vehicle-borne explosive attacks are tactics that are not beyond the reach of any modestly funded and committed terrorist group. (Heritage)