After the end of the Cold War, the new European and global security environment has undergone profound changes. In the new security environment, international terrorism represents one of the main threats at a global level, the others being the proliferation of chemical, atomic, and bacteriological weapons of destruction and organized crime. Terrorism is difficult to define. Although it is an important issue today, there is no unanimously accepted scientific definition. There have been numerous attempts to define terrorism that complement each other. The paper focuses on Islamic terrorism which has undergone a certain evolution over time and has proliferated in recent years. Nowadays, it has some particularities which will be presented in the paper; they reflect the specificity of this phenomenon with a view to understanding its origin, the purposes, and its consequences. Samuel P. Huntington’s theory of the Clash of Civilization and Eric Neumayer and Thomas Plümper’s theory of the Strategic Logic of International Terrorism provide useful explanations. The aim of the paper is to examine the evolution of this phenomenon after 9/11 2001, which represents a turning point in the evolution of international terrorism.