Russia was the first to order a partial mobilisation of its armies on 24–25 July, and when on 28 July Austria-Hungary declared war on Serbia, Russia declared general mobilisation on 30 July. Germany presented an ultimatum to Russia to demobilise, and when this was refused, declared war on Russia on 1 August. Being outnumbered on the Eastern Front, Russia urged its Triple Entente ally France to open up a second front in the west.
Japan entered the war on the side of the Allies on 23 August 1914, seizing the opportunity of Germany's distraction with the European War to expand its sphere of influence in China and the Pacific.
Over forty years earlier in 1870, the Franco-Prussian War had ended the Second French Empire and France had ceded the provinces of Alsace-Lorraine to a unified Germany. Bitterness over that defeat and the determination to retake Alsace-Lorraine made the acceptance of Russia's plea for help an easy choice, so France began full mobilisation on 1 August and, on 3 August, Germany declared war on France. The border between France and Germany was heavily fortified on both sides so, according to the Schlieffen Plan, Germany then invaded neutral Belgium and Luxembourg before moving towards France from the north, leading the United Kingdom to declare war on Germany on 4 August due to their violation of Belgian neutrality.
By the end of the war or soon after, the German Empire, Russian Empire, Austro-Hungarian Empire and the Ottoman Empire ceased to exist. National borders were redrawn, with nine independent nations restored or created, and Germany's colonies were parcelled out among the victors. During the Paris Peace Conference of 1919, the Big Four powers (Britain, France, the United States and Italy) imposed their terms in a series of treaties. The League of Nations was formed with the aim of preventing any repetition of such a conflict. This effort failed, and economic depression, renewed nationalism, weakened successor states, and feelings of humiliation (particularly in Germany) eventually contributed to the start of World War II.
From the time of its start until the approach of World War II, the First World War was called simply the World War or the Great War and thereafter the First World War or World War I. At the time, it was also sometimes called "the war to end war" or "the war to end all wars" due to its then-unparalleled scale and devastation.
In Canada, Maclean's magazine in October 1914 wrote, "Some wars name themselves. This is the Great War." During the interwar period (1918–1939), the war was most often called the World War and the Great War in English-speaking countries.
The term "First World War" was first used in September 1914 by the German biologist and philosopher Ernst Haeckel, who claimed that "there is no doubt that the course and character of the feared 'European War' ... will become the first world war in the full sense of the word," citing a wire service report in The Indianapolis Star on 20 September 1914. After the onset of the Second World War in 1939, the terms World War I or the First World War became standard, with British and Canadian historians favouring the First World War, and Americans World War I.
In the introduction to his book, Waterloo in 100 Objects, historian Gareth Glover states: "This opening statement will cause some bewilderment to many who have grown up with the appellation of the Great War firmly applied to the 1914–18 First World War. But to anyone living before 1918, the title of the Great War was applied to the Revolutionary and Napoleonic wars in which Britain fought France almost continuously for twenty-two years from 1793 to 1815." In 1911, the historian John Holland Rose published a book titled William Pitt and the Great War.