What is the Real Threat?
The threat comes primarily from nation states with the will, motive, and technology to launch attacks against the United States. In fact, nations that do not possess a powerful military are probably more likely to choose this form of attack because of its much lower cost of implementation (think "asymmetric warfare").
The most obvious avenues of attack would be against our nation’s critical infrastructure, so the government formed a body to analyze any threats against our infrastructure. This body, originally known as the President’s Critical Infrastructure Protection Board (PCCIB), is now known as the National Infrastructure Advisory Council (NIAC) and operates within the U.S. Department of Homeland Security.
This council provides the President with advice on the security of the critical infrastructure sectors and their information systems. Over time, the sectors identified as belonging to the critical infrastructure have changed. The current definitions, as specified in the National Infrastructure Protection Plan, are shown in Figure 1.
Figure 1 Sector-specific agencies and HSPD-7 assigned CI/KR sectors