There is a large number of countries in the world that have experienced and been victims of terrorist attacks. Terrorism is one of the main global problems that is difficult to define and determine its goals and causes in order for it to be prevented in a timely manner. Every terrorist attack causes shock, disbelief and fear, as well as numerous condemnatory reactions to the act itself. But the event that took place on September 11, 2001, left consequences not only on the United States but on the whole flower. The 9/11 terrorist attack is one of the most significant events of the 21st century, which showed that even the United States, as one of the most powerful countries in the world, is not immune to terrorism and its consequences. By hijacking four American planes, nineteen attackers turned them into the biggest suicide bombs in history. Condemning reactions to this event came from all over the world. The aim of this paper is to show in more detail what happened on September 11, 2001, who is behind the attack on the United States, what are the consequences, and what is the international reaction to the event itself.