Combating a Modern Hydra: Al Qaeda and the Global War on Terrorism is number eight in the Combat Studies Institute's Global War on Terrorism (GWOT) Occasional Paper series. This work resulted from discussions at Fort Leavenworth about the nature of the enemy facing the United States and its allies since 11 September 2001. Osama bin Laden and his terrorist network had been present at some level in the national and international consciousness since the late 1990s. The events of 11 September 2001 and subsequent global operations taken against Al Qaeda have brought this group to the forefront of the GWOT. While successes have been achieved in the GWOT, the enemy has proven to be resilient and adaptive. This study by Mr. Sean Kalic, of the Department of Military History, US Army Command and General Staff College, examines modern transnational terrorism from the 1960s to the present day, with special emphasis on the adaptation Al Qaeda and other nonstate actors have taken in response to the actions of the United States and its allies. This work provides a cautionary warning about the likelihood Al Qaeda will continue to survive and execute missions in the current operating environment. Mr. Kalic synthesizes much of the pertinent literature and offers insights into the actions taken to fight terrorists. Most importantly, he advises a continual reevaluation of the threat, based on Al Qaeda's flexibility, resiliency, and adaptability. Officers and soldiers who have recently served in operations against the terrorist worldwide will certainly see utility here. As the US Army continues its efforts in combating terrorists, the thoughts found in this narrative are well worth considering.