On 25 July, the Russian government issued orders for the 'period preparatory to war'; after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved of the military districts nearest to Austria, including Kiev, Kazan, Odessa and Moscow. General Russian mobilisation was announced on the evening of 30 July; on the 31st, Austria-Hungary and Germany did the same, while Germany demanded Russia demobilise within 12 hours. When Russia failed to comply, Germany declared war on 1 August, with Austria-Hungary following suit on 6th; France ordered full mobilisation in support of Russia on 2 August. French entry into the war stemmed from a combination of the desire to regain the provinces of Alsace-Lorraine ceded after the 1870–1871 Franco-Prussian War, concern at Germany's increasing power and military commitments agreed with Russia.
German strategy for a war on two fronts against France and Russia was to concentrate the bulk of their army in the West to defeat France within four weeks, then shift forces to the East before Russia could fully mobilise; this was later known as the Schlieffen Plan. On 2 August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France. When this was refused, German forces entered Belgium early on the morning of 3 August and declared war with France the same day; the Belgian government invoked the 1839 Treaty of London and in compliance with its obligations under this, Britain declared war on Germany on 4 August. On 12 August, Britain and France also declared war on Austria-Hungary; on the 23rd, the Empire of Japan joined the Allied Powers, seizing the opportunity to expand its sphere of influence by capturing German possessions in China and the Pacific. On 24 August, Serbia won a major victory over the Austro-Hungarians at the Battle of Cer.
In November 1914, the Ottoman Empire joined the Central Powers, opening fronts in the Caucasus, Mesopotamia and the Sinai Peninsula. In 1915, Italy joined the Allied Powers and Bulgaria joined the Central Powers. Romania joined the Allied Powers in 1916. After the sinking of seven US merchant ships by German submarines, and the revelation that the Germans were trying to incite Mexico to make war on the United States, the US declared war on Germany on 6 April 1917.
By the end of the war or soon after, the German Empire, Russian Empire, Austro-Hungarian Empire and Ottoman Empire ceased to exist. National borders were redrawn 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, however, as weakened successor states, renewed nationalism, economic depression, and feelings of humiliation (particularly in Germany) eventually contributed to the start of World War II.
The term "First World War" was first used in September 1914 by 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.
Prior to World War II, the events of 1914–1918 were generally known as the Great War or simply the World War. Contemporary Europeans also referred to it as "the war to end war" or "the war to end all wars" due to their perception of its then-unparalleled scale and devastation. After World War II began in 1939, the terms became more standard, with British Empire historians, including Canadians, favouring "The First World War" and Americans "World War I".
In October 1914, the Canadian magazine Maclean's wrote, "Some wars name themselves. This is the Great War." However, while certainly accurate in Canada, this was not so even in Britain; historian John Holland Rose's 1911 account of the 1793–1815 wars against France was titled William Pitt and the Great War. This was validated by Gareth Glover's 2015 book, Waterloo in 100 Objects, in which he 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 Germany, "The Great War" was historically used for the 1618–1648 Thirty Years' War, also known as the "Great German War" or "Great Schism". One of the longest and most destructive conflicts in human history, it resulted in eight million fatalities from military action, violence, famine and plague, the vast majority of them in the German states of the Holy Roman Empire. In terms of proportional German casualties and destruction, it was only surpassed by the period from January to May 1945; its enduring visibility is partly the result of 19th-century Pan-Germanism, as an example of the dangers of a divided Germany and a driver in the 1871 creation of the Deutsches Kaiserreich or German Empire. Regardless of terminology, the Thirty Years' War remains the single greatest war trauma in German memory, as demonstrated in debates over naming conventions during the centenary of 1914–1918.