1914 Russian poster depicting the Triple Entente
When the war began in 1914, the Central Powers were opposed by the Triple Entente, formed in 1907 by the British Empire, the Russian Empire and the French Third Republic.
Fighting commenced when Austria invaded Serbia on 28 July 1914, purportedly in response to the Assassination of Archduke Franz Ferdinand, heir to Emperor Franz Joseph; this brought Serbia's ally Montenegro into the war on 8 August and it attacked the Austrian naval base at Cattaro, modern Kotor. At the same time, German troops entered neutral Belgium and Luxembourg as dictated by the Schlieffen Plan; over 95% of Belgium was occupied but the Belgian Army held their lines on the Yser Front throughout the war. This allowed Belgium to be treated as an Ally, in contrast to Luxembourg which retained control over domestic affairs but was occupied by the German military.
In the East, between 7-9 August the Russians entered German East Prussia on 7 August, Austrian Eastern Galicia. Japan joined the Entente by declaring war on Germany on 23 August, then Austria on 25 August. On 2 September, Japanese forces surrounded the German Treaty Port of Tsingtao (now Qingdao) in China and occupied German colonies in the Pacific, including the Mariana, Caroline, and Marshall Islands.
Despite its membership of the Triple Alliance, Italy remained neutral until 23 May 1915 when it joined the Entente, declaring war on Austria but not Germany. On 17 January 1916, Montenegro capitulated and left the Entente; this was offset when Germany declared war on Portugal in March 1916, while Romania commenced hostilities against Austria on 27 August.
On 6 April 1917, the United States entered the war as a co-belligerent, along with the associated allies of Liberia, Siam and Greece. After the 1917 October Revolution, Russia left the Entente and and agreed a separate peace with the Central Powers with the signing of the Treaty of Brest-Litovsk on 3 March 1918. Romania was forced to do the same in the May 1918 Treaty of Bucharest but on 10 November, it repudiated the Treaty and once more declared war on the Central Powers.
These changes meant the Allies who negotiated the Treaty of Versailles in 1919 included France, Britain, Italy, Japan and the US; Part One of the Treaty agreed to the establishment of the League of Nations on 25 January 1919. This came into being on 16 January 1920 with Britain, France, Italy and Japan as permanent members of the Executive Council; the US Senate voted against ratification of the Treaty of Versailles on 19 March, thus preventing American participation.