When is it acceptable to invade another country to prevent an attack on your own? What level of threat is required to act? What kind of proof is needed for the action to achieve legitimacy?

Joining us to discuss this issue are David Phillips, Kerry advisor on foreign policy and a senior fellow at the Council on Foreign Relations, and Richard Perle, a resident fellow at the American Enterprise who served as an assistant secretary of defense in the Reagan administration and as chairman of the Defense Policy Board under President George Bush.

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In 2002, during the buildup to the invasion of Iraq, the Bush administration issued its "National Security Strategy of the United States of America” which outlined its philosophy of pre-emptive war.

From Section 5 of the document: The United States has long maintained the option of preemptive actions to counter a sufficient threat to our national security. The greater the threat, the greater is the risk of inaction— and the more compelling the case for taking anticipatory action to defend ourselves, even if uncertainty remains as to the time and place of the enemy’s attack. To forestall or prevent such hostile acts by our adversaries, the United States will, if necessary, act preemptively.

This was said by some commentators to represent a major shift in American foreign policy. John Kerry wouldn’t seem to agree with that assessment. As he said in last week’s debate, “The president always has the right and always has had the right for pre-emptive strike. . . No president through all of American history has ever ceded and nor would I the right to pre-empt in any way necessary to protect the United States of America.”

From there the two candidates seem to part company. Kerry’s next statement, “But if and when you do it, Jim, you've got to do it in a way that passes the test. That passes the global test where your countrymen, your people understand fully why you're doing what you're doing. And you can prove to the world that you did it for legitimate reasons,” has been attacked by the Bush campaign as allowing other countries to veto U.S. military action and is the basis for a new anti-Kerry ad entitled “Global Test.”