World War II, which lasted from 1939 to 1945, was one of the largest and most impactful conflicts in human history. This conflict involved many countries around the world and changed the global political and social map. In Europe, World War II began on September 1, 1939, when Germany, led by Adolf Hitler, attacked Poland. This attack prompted a swift reaction from Britain and France, who declared war on Germany. The conflict was divided into two main alliances: the Allies and the Axis. The Allies consisted of countries such as the United States, Soviet Union, Britain, and China, while the Axis included Germany, Italy, and Japan. One factor causing tensions is the rise of totalitarianism in Europe and Asia. Germany carried out aggressive territorial expansion, including the annexation of Austria and Czechoslovakia. In the Pacific, Japan attempted to expand its power by attacking the US Navy facilities at Pearl Harbor on December 7, 1941, which brought the United States officially into the war. The military strategies used during the war were very diverse, including Blitzkrieg by Germany, which relied on rapid and tactical attacks, as well as guerrilla warfare strategies by Allied forces in enemy-held territory. World War II had a devastating impact, with an estimated 70 to 85 million people losing their lives. The Holocaust, which was the genocide of six million Jews by the Nazi regime, was one of the greatest tragedies to occur during this conflict. In addition, the use of nuclear weapons by the US in Hiroshima and Nagasaki in 1945 hastened the end of the war while demonstrating the potential for mass destruction that modern technology had. After World War II, the world experienced major changes. The UN organization was founded to prevent similar conflicts in the future and maintain peace. This war also encouraged the development of technology, diplomacy and the global economy. The cold war between the Soviet Union and the United States emerged as a result of sharp ideological differences after the end of the war. The social, political, and economic impacts of World War II continue to be felt to this day. The devastated countries of Europe slowly rebuilt themselves, and the process of decolonization began in many parts of the world, resulting in the creation of new nations. World War II was not only an event in history, but also a profound lesson in the consequences of military aggression and the importance of international cooperation.