World War I alliances included the Triple Entente and the Central Powers. The Triple Entente included France, Britain and the Russian Empire. France and Russia had been military allies against Germany since the early 1890s. In April of 1904, Britain concluded a series of agreements with France regarding the division of their colonial interests and the resolution of associated conflicts. These agreements are known as the Entente Cordiale. While in these agreements Britain did not explicitly pledge British support to the French in the case of German attack, it implicitly abandoned an earlier idea of seeking an agreement with Germany in favor of rapprochement with France and Russia. In 1907, Britain signed an agreement with the Russian Empire regarding respective spheres of influence in Asia.
Later in the war, Italy and Japan joined the alliance with France, Britain, and Russia; from this point, the Triple Entente became known as the Allied Powers. Although previously allied with Germany, in 1915 Italy joined the war on the side of the Triple Entente. Serbia fought on the same side since 1914. Germany invaded Belgium in 1914 and, as a result, Belgium became part of the Allied coalition. Japan also entered the war in 1914 on the side of the Triple Entente. Romania joined the Allied Powers in 1916, and the US became part of the Allied coalition in 1917 when it declared war on Germany. Russia, on the other hand, ended its participation when the new Bolshevik government signed the separate Brest-Litovsk peace treaty with Germany and its allies in 1918.
On the other side in the war were the Central Powers, including the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, and Bulgaria.
Germany and Italy had been allied with the Austro-Hungarian Empire since 1882, and the grouping came to be called the Triple Alliance, but Italy withdrew from this alliance prior to the start of World War I. The Ottoman Empire, on the other hand, established close relationships with Germany during the early twentieth century and in 1914 joined the Central Powers and entered the war as a German and Austrian ally. Bulgaria followed in 1915.