A Study on Legality of U.S. Military Responses to 9. 11 TerrorPark, Won-SeogThe military responses of U.S. and U.K. to 9. 11 terror raise several important issues under international law regarding the use of force. Although the United States has asserted a lawful exercise of self defense in Article 51 of the U.N. Charter, it is not clear whether the requirement of an "armed attack" has been met. This paper will examine first the general principles of international law regarding use of force and its exceptions. Second, it analyzes the present regime of international law and its application to the use of military force against terrorist groups. Among others are some particular issues considered: (1) is there an "armed attack" within the meaning of Article of 51 when a terrorist organization provides funds and other support to individual for terrorism? (2) what are the circumstances under which a state can use military force against a terrorist organization physically located within the boundaries of another state? (3) what are the limitations and restrictions upon use of military force against a terrorist organization? (4) What level of support will be required for the harboring state to be responsible for the activities of the terror groups where they are located? Finally, it suggests the persuasive arguments that 9. 11 terror constitutes an "armed attack" against the United States.