Yet there is another nuclear threat to the U.S. homeland that could be posed by terrorists that is much less well-known - to our collective peril. This other nuclear threat is just as plausible and equally credible when compared to the threat of a weapon smuggled into the United States. Compared to a smuggled nuclear weapon detonated in New York, D.C. or Los Angeles, this other nuclear threat is potentially far more catastrophic: instead of a single city, it could threaten the entire nation's survival.

But the DHS and their institutional advisers are so fixated on the "conventional wisdom" of the threat from a nuclear bomb smuggled in that they are doing far too little to detect and prevent nuclear terrorists and their state sponsors from executing an electromagnetic pulse (EMP) attack on the United States.

A high-altitude EMP results from the detonation of a nuclear warhead at altitudes above 25 miles over the Earth's surface, and covers the area within line-of-sight from the bomb. The immediate effects of EMP are disruption of, and damage to, electronic systems that are indispensable to the operation of critical national infrastructures - the electric power grid, wired and cell telephone systems, fuel handling, land and air transportation, government operations, banking and finance, food storage and distribution, and water treatment and supply - that sustain our economy, military power and civilian population.

Our vulnerability to EMP attack is increasing daily as our dependence on electronics continues to grow. The impact of EMP is asymmetric in relation to potential antagonists who are less dependent on modern electronics.

The recovery from EMP attack of any one of the key national infrastructures is dependent on the recovery of others, in the same way their normal operations are interdependent. The longer the outage, the more problematic and uncertain the recovery would be. It is possible for the functional outages to become mutually reinforcing, until at some point the degradation of critical national infrastructures could irreversibly affect U.S. ability to support its population and sustain its role in the world.

Several potential adversaries have the knowledge and the resources to attack the United States with a high-altitude nuclear-weapon-generated EMP, and others appear to be pursuing efforts to obtain that capability. A determined adversary could carry out an EMP attack without having the high level of technical sophistication of a major nation.

One scenario of special concern is an EMP attack against the United States launched from an ordinary freighter off the U.S. coast using a short- or medium-range missile to loft a nuclear warhead to high-altitude (such missiles are readily available on the world armaments black market).

Terrorists sponsored by a hostile state could try to launch such an attack without revealing the sponsors' identity. Iran, the world's leading sponsor of international terrorism, has practiced launching a mobile ballistic missile from a vessel in the Caspian Sea. Iran has also tested high-altitude explosions of its Shahab-III ballistic missile, a test mode consistent with EMP attack. Iranian military writings explicitly discuss a nuclear EMP attack that would destroy the United States. Connecting the dots is not difficult.

Designs for missile-launched nuclear weapons may have been illicitly trafficked for at least a quarter-century. Recently, as reported in the press, United Nations investigators found the design for an advanced nuclear weapon, miniaturized to fit ballistic missiles currently in the inventory of Iran, North Korea and other potentially hostile states, was in the possession of Swiss nationals affiliated with the A.Q. Khan nuclear proliferation network.

This suggests that additional nuclear weapon designs may also be in the possession of hostile states and of states that sponsor terrorism. However, even a primitive, low-yield modern day "entry-level" nuclear weapon could be used to conduct an EMP attack.

Why is the Department of Homeland Security moving aggressively to protect America's cities and seaports from nuclear terrorists smuggling in a nuclear weapon but overlooking the possibility of EMP attack? Their assumption is that if terrorists acquire a nuclear weapon, they would certainly prefer to detonate it in a major metropolitan area, rather than attack an entire seaboard or even the whole nation with EMP.

The assumption that a nuclear weapon would be used against us in only one way is unwarranted, as an EMP attack offers some significant advantages over smuggling.

Smuggling a nuclear weapon into a U.S. city is risky, and becoming increasingly so, as homeland security measures improve. Significant investments are being made in measures to defeat such attempts. In contrast, an EMP attack using a missile launched from a ship outside U.S.-controlled waters eliminates most of the operational hazards of smuggling a nuclear weapon into a U.S. port or city. Moreover, it offers less opportunity for detection, less risk of weapon seizure, less risk of crewmember defection, greater difficulty for the United States in conducting forensic analysis to determine who sponsored the attack, less certainty of prompt retaliation and greater long-term, potentially catastrophic consequences for the nation.

Indeed, EMP attack is the only nuclear option where one or two nuclear weapons could gravely damage the entire United States, and give terrorism a large-scale victory from attacking the U.S.

While an EMP attack on our critical national infrastructures is one of the most serious terrorist and hostile state threats facing our nation, the United States need not be vulnerable to the catastrophic consequences of such an attack. The nation owes the recent progress made toward addressing EMP to the leadership of Rep. Roscoe G. Bartlett, Maryland Republican, and one of the few scientists in Congress, whose concern led him to initiate legislation that established the commission that I chair to address the EMP threat.

The EMP Commission has proposed recommendations that, over a five-year period, at reasonable cost, would enable the United States to prepare, train, protect and recover its infrastructures against EMP attack. This same plan would also help protect critical national infrastructures from other threats, including cyber attack, sabotage and natural disasters such as very large geomagnetic storms and major hurricanes.

Continued failure to protect the United States from EMP invites attack.

More information, including the EMP Commission's report on the EMP threat to the Critical National Infrastructure, can be found at www.Empcommission.org.