The reason being is in the beginning of the war Russia was allied with Serbia while Germany was allied with Austria-Hungary. When Austria-Hungary's Archduke was assassinated while in Serbia they declared war on the Serbians. True to Russia's war saying that they would help Serbia in time of war declared war on Austria-Hungary, Because of the alliance between Austria-Hungary and Germany, Germany declared war on both Russia and Serbia which then just escalated from there dragging the nations of France, Britain, and Italy into the war. so then you had one alliance consisting of France, Britain, and Russia forming the triple entente, and the Triple Alliance on the other consisting of Austria-Hungary, Germany, and Italy.
