A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe. By 1914, the great powers of Europe were divided into two coalitions: the Triple Entente—consisting of France, Russia and Britain—and the Triple Alliance of Germany, Austria-Hungary and Italy (the Triple Alliance was primarily defensive in nature, allowing Italy to stay out of the war in 1914). Russia felt it necessary to back Serbia, on 25 July issuing orders for the 'period preparatory to war', and after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved of the military districts nearest to Austria. 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 in support of Austria-Hungary, with Austria-Hungary following suit on 6th; France ordered full mobilisation in support of Russia on 2 August.
German strategy for a war on two fronts against France and Russia was to concentrate the bulk of its 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 invaded 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, Japan sided with the Entente, seizing the opportunity to expand its sphere of influence by capturing German possessions in China and the Pacific. The war was fought in and drew upon each powers' colonial empires as well, spreading the conflict across the globe. The Entente and its allies would eventually become known as the Allied Powers, while the grouping of Austria-Hungary and Germany would become known as the Central Powers.
As a result of the war, the Russian, German, Austro-Hungarian and Ottoman empires were replaced by new states, based on nationalities. The Four Powers of Britain, France, the US and Italy, imposed their terms in a series of treaties agreed at the 1919 Paris Peace Conference. The formation of the League of Nations was intended to prevent another world war, but for various reasons failed to do so. The central role of the Nazi Party in World War II led to a focus on how the Treaty of Versailles affected Germany, but the peace also transformed large parts of Europe and the Middle East, in ways that remain relevant today.
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.