World wars refer to large-scale conflicts that involve many countries across the globe. The two most significant world wars in history are World War I and World War II. World War I occurred from 1914 to 1918, primarily involving European nations, and was triggered by political tensions and alliances. It resulted in millions of casualties and significant changes in national borders. World War II took place from 1939 to 1945 and was even more widespread, involving countries from all continents. It began with the invasion of Poland by Nazi Germany and included major powers like the United States, Soviet Union, and United Kingdom. The war ended with the defeat of the Axis powers and led to the establishment of the United Nations to promote peace and prevent future conflicts.