**Reasons:
1. Global Scale:** World War II was a global conflict, involving countries from all continents and many oceans. The war affected people in various regions, with battles taking place in Europe, Asia, Africa, and the Pacific.
2. Large-Scale Destruction:** The war caused unprecedented destruction on a large scale. Entire cities were destroyed, millions of people died, and significant cultural and historical landmarks were lost. Many countries struggled with massive civilian losses and infrastructure damage.
3. Ideological Conflict:** World War II was driven by deep ideological differences between nations. Fascism, communism, democracy, and other competing ideologies clashed on a global scale, fundamentally shaping post-war geopolitical landscapes.
4. Technological Advancements:** The war accelerated technological advancements, especially in military sectors. Innovations like the atomic bomb, jet engines, rockets, and submarines had profound impacts on international conflicts and military strategies.
5. Post-War Restructuring:** The end of the war led to political, territorial, and economic restructuring across the world. Countries underwent decolonization processes, new international organizations emerged (like the United Nations), and global alliances altered, leading to changes in the balance of power.
6. Emergence of Superpowers:** After World War II, the United States and the Soviet Union emerged as dominant global powers. Their rivalry, known as the Cold War, defined international relations for the following decades.
7. Cultural Impact:** The war profoundly affected cultures, societies, and arts worldwide. Post-war movements embraced existentialism, anti-war activism, human rights, and other social consciousness, influencing literature, philosophy, and artistic expressions around the world.
In short, World War II's extensive international participation, immense destruction, ideological clashes, technological innovations, post-war reshaping, the prominence of superpowers, and cultural impact make it the war with the most extensive international effects on human history.