The terrorism threat to the United States over the next five years will be driven by instability in the Middle East and Africa, persistent challenges to border security and increasing Internet savvy, says a new intelligence assessment from the U.S. Homeland Security Department. Chemical, biological, radiological and nuclear attacks are considered the most dangerous threats that could be carried out against the U.S. but are also the most unlikely because it is so difficult for al-Qaida and similar groups to acquire the materials needed to carry out such plots.