<human>:  What was the first major war of the 20th century?
<bot>: The First World War started in 1914 and lasted until 1918. It was fought between the Allied Powers, which included the United Kingdom, France, the United States, Russia, Italy, Japan, and others, and the Central Powers, which included Germany, Austria-Hungary, Bulgaria, Turkey, and others. The war resulted in the deaths of more than 9 million soldiers, including millions of civilians.