The President of the United States has no greater responsibility than protecting the American people from threats, both foreign and domestic.

As the nature of the threats to the United States changes, so must the nation's approach to its defense. In situations where the evidence demonstrates overwhelmingly that behavioral trends, capability, and motives all point to imminent threat, it may be necessary for the President to attack preemptively.

The reality of international life in the 21st century is that nations or organizations that wish to challenge America or Western powers increasingly are seeking weapons of mass destruction to achieve their political objectives. The only effective response may be to destroy those capabilities before they are used. The tenet of traditional, customary international law that allows for this preventive or preemptive action is "anticipatory self-defense."

Historically, the United States has asserted its right to anticipatory self-defense:

• In 1962, President John Kennedy ordered a blockade of Cuba during the Cuban missile crisis.
• During the 1980s, President Ronald Reagan invoked this right at least twice: first, in 1983, ordering an invasion of Grenada, and again in 1986, ordering the bombing of terrorist sites in Libya.

Learning From the September 11 Attacks

Complacency is not acceptable. U.S. authorities knew of the threat posed by Osama bin Laden, yet did nothing.

1. Deterrence alone is not sufficient to suppress aggression.
2. Attacks can occur with little or no warning. In this world of drastically shortened time lines, it is essential that the President have the authority to act decisively, in short order, to defeat aggressors when a preponderance of information points to a threat of imminent attack. While the President did not know the September 11 attacks would happen, there was ample evidence that threats to the United States would likely emerge from Afghanistan.
3. The use of a weapon of mass destruction is reasonably likely. Hostile entities increasingly view weapons of mass destruction as political assets.
4. A deadly synergy is created when hostile state and non-state agents conspire. The reality of the 21st century is that a state like Iraq can harness its resources to develop a weapon of mass destruction and collude with non-state actors to deliver that weapon.
5. The future envisioned by America's enemies is incompatible with U.S. security. On September 11, the idea that hostile regimes and the United States could simultaneously pursue their respective interests lost all credibility. It was clear that America's enemies were willing to use unprovoked violence to achieve their objectives. The United States could no longer postpone acting against terrorists and nations that support them.

This WebMemo is excerpted from Jack Spencer's Backgrounder: Presidential Authority in the War on Terrorism: Iraq and Beyond.Full footnotes and analysis are available there.