World War I: Start of the Battle of Lorraine, an unsuccessful French offensive.

World War I, or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Major theatres of war included Europe and the Middle East, and well as parts of Africa and the Asia-Pacific. One of the deadliest conflicts in history, World War I resulted in an estimated 15 to 22 million deaths, including in mass war crimes against civilians and genocides. The conflict saw important developments in weaponry, including the first large-scale use of machine guns, artillery, aircraft, chemical weapons, and tanks, and was a major factor in the deadly Spanish flu pandemic.