The Second World War was a global conflict that lasted from 1939 to 1945. It involved many countries, primarily divided into two main groups: the Allies, which included nations like the United States, United Kingdom, and Soviet Union, and the Axis Powers, led by Germany, Italy, and Japan. The war began when Germany invaded Poland, prompting Britain and France to declare war. The conflict resulted in significant events, including the Holocaust and the use of atomic bombs on Hiroshima and Nagasaki. It caused immense destruction and loss of life, with an estimated 70-85 million fatalities. The war ended with the unconditional surrender of the Axis Powers in 1945, leading to the establishment of the United Nations to promote peace and prevent future conflicts.