A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe. By July 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 only defensive in nature, allowing Italy to stay out of the war until April 1915, when it joined the Allied Powers after its relations with Austria-Hungary deteriorated). Russia felt it necessary to back Serbia and, after Austria-Hungary shelled the Serbian capital of Belgrade on the 28July, approved partial mobilisation. Full Russian mobilisation was announced on the evening of 30July; on the 31st, Austria-Hungary and Germany did the same, while Germany demanded Russia demobilise within twelve hours. When Russia failed to comply, Germany declared war on Russia on 1August in support of Austria-Hungary, with Austria-Hungary following suit on 6August; France ordered full mobilisation in support of Russia on 2August.
German strategy for a war on two fronts against France and Russia was to rapidly concentrate the bulk of its army in the West to defeat France within six weeks, then shift forces to the East before Russia could fully mobilise; this was later known as the Schlieffen Plan. On 2August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France. When this was refused, German forces invaded Belgium on 3August and declared war on 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 4August. On 12 August, Britain and France also declared war on Austria-Hungary; on the 23August, Japan sided with Britain, seizing German possessions in China and the Pacific. In November 1914, the Ottoman Empire entered the war on the side of the Central Powers, opening fronts in the Caucasus, Mesopotamia, and the Sinai Peninsula. The war was fought in and drew upon each power's colonial empire as well, spreading the conflict to Africa and across the globe. The Entente and its allies would eventually become known as the Allied Powers, while the grouping of Austria-Hungary, Germany and their allies would become known as the Central Powers.
The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917 (the Eastern Front, by contrast, was marked by much greater exchanges of territory). In 1915, Italy joined the Allied Powers and opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans. The United States initially remained neutral, though even while neutral it became an important supplier of war materiel to the Allies. Eventually, after the sinking of American merchant ships by German submarines, the declaration by Germany that its navy would resume unrestricted attacks on neutral shipping, and the revelation that Germany was trying to incite Mexico to make war on the United States, the U.S. declared war on Germany on 6April 1917. Trained American forces would not begin arriving at the front in large numbers until mid-1918, but ultimately the American Expeditionary Force would reach some two million troops.
World War I was a significant turning point in the political, cultural, economic, and social climate of the world. It is considered to mark the end of the Second Industrial Revolution and the Pax Britannica. The war and its immediate aftermath sparked numerous revolutions and uprisings. The Big Four (Britain, France, the United States, and Italy) imposed their terms on the defeated powers in a series of treaties agreed at the 1919 Paris Peace Conference, the most well known being the German peace treaty—the Treaty of Versailles. Ultimately, as a result of the war the Austro-Hungarian, German, Ottoman, and Russian Empires ceased to exist, with numerous new states created from their remains. However, despite the conclusive Allied victory (and the creation of the League of Nations during the Peace Conference, intended to prevent future wars), a second world war would follow just over twenty years later.
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. In October 1914, the Canadian magazine Maclean's wrote, "Some wars name themselves. This is the Great 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 WarII began in 1939, the terms became more standard, with British Empire historians, including Canadians, favouring "The First World War" and Americans "World WarI".