"A Military Guide to Terrorism in the Twenty-First Century" is a capstone reference guide prepared under the direction of the U.S. Army Training and Doctrine Command (TRADOC), TRADOC G2 as a capstone reference guide on terrorism. This guide addresses foreign and domestic threats against the United States of America in a contemporary operational environment (COE). This informational handbook supports institutional training, professional military education, and operational missions for U.S. military forces in the War on Terrorism (WOT). This document provides an introduction to the nature of terrorism and recognition of terrorist threats to U.S. military forces. A common situational awareness by U.S. military forces considers three principal venues for armed forces: forces that are deployed, forces that are in transit to or from an operational mission, and forces that are primarily installation or institution support. Neither a counterterrorism directive nor antiterrorism manual, this handbook complements but does not replace Army training and intelligence products on terrorism. A selected bibliography presents citations for detailed study of specific terrorism topics.