In this Marshall Center Paper, Professor Michael Schmitt explores the legality of the attacks against Al Qaeda and the Taliban under the jus ad bellum, that component of international law that governs when it is that a State may resort to force as an instrument of national policy. Although States have conducted military counter-terrorist operations in the past, the scale and scope of Operation Enduring Freedom may well signal a sea change in strategies to defend against terrorism. This Paper explores the normative limit on counter-terrorist operations. Specifically, under what circumstances can a victim State react forcibly to an act of terrorism? Against whom? When? With what degree of severity? And for how long? Professor Schmitt concludes that the attacks against Al Qaeda were legitimate exercises of the rights of individual and collective defense. They were necessary and proportional, and once the Taliban refused to comply with US and United Nations demands to turn over the terrorists located in Afghanistan, it was legally appropriate for coalition forces to enter the country for the purpose of putting an end to the ongoing Al Qaeda terrorist campaign.