World War One or the Great War was fought from 1914 to 1918 and was eventually won by the Allied Powers.
Those fighting for the Allied Powers originally was the British Empire, the French Third Republic, Russia, the Empire of Japan, the Kingdom of Serbia and Belgium.
In 1915 Italy joined the campaign, followed by Romania in 1916 and then Greece in 1917.
Also in 1917, the German Foreign Minister invited Mexico to join the war as an ally. In return Germany would help Mexico recover land from the United States (Texas, New Mexico and Arizona). Mexico refused and the exchange was made public, the United States viewed this as a just cause for war and joined the Allied efforts.
In that same year (1917) Russia withdrew from combat mostly due to a looming civil war which then started in 1918. The Russian withdrawal allowed for the final structure of the alliance, which was based on five Great Powers: United Kingdom, United States, France, Italy and Japan.
It was with this alliance that the Allied forces could gain control and prevent the enemy from mounting a successful defence. Finally in the winter of 1918 the Central Powers were defeated.
The Central Powers consisted of the German Empire, Austria-Hungary, the Ottoman Empire and, from 1915, Bulgaria.