World War I: Adhering to the terms of the Treaty of London, the United Kingdom declared war on Germany in response to the latter's invasion of Belgium.

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.