World War 1, also known as the Great War or the First World War, involved two main sides - the Allied Powers and the Central Powers. The Central Powers included Germany, the Ottoman Empire, and the Austro-Hungarian Empire, while the Allied Powers included Britain, France, the Russian Empire, Japan, and the United States. The main reasons for the war included militarism, formation of alliances through treaties, imperialism, and nationalism. The immediate cause of the war was the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist in June 1914. This led to Austria-Hungary declaring war on Serbia, triggering a chain of alliances that culminated in the start of World War 1 on July 28, 1914.