On 25 July Russia began mobilisation and on 28 July the Austro-Hungarians declared war on Serbia. 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. Back in 1870, the Franco-Prussian War had ended the Second French Empire and 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. After the German march on Paris was halted in the Battle of the Marne, what became known as the
Western Front settled into a
battle of attrition, with a
trench line that changed little until 1917. On the
Eastern Front, the Russian army led a successful campaign against the Austro-Hungarians, but the Germans stopped its
invasion of East Prussia in the battles of Tannenberg and the Masurian Lakes. In November 1914, the Ottoman Empire joined the Central Powers, opening fronts in the
Mesopotamia and the
Sinai. In 1915, Italy joined the Allies and Bulgaria joined the Central Powers;
Romania joined the Allies in 1916, as did the United States in 1917.
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 several independent nations restored or created, and
Germany's colonies were parceled out among the victors. During the
Paris Peace Conference of 1919, the
Big Four (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.