In 1914, the six most powerful countries in Europe were Britain, France, Russia, Germany, Austria-Hungary and Italy, divided into two opposing alliances. Russia, along with Britain and France, was part of the Triple Entente, formed in 1907. It was the first among the nations to mobilize troops against Germany.
Russia was by far the largest of all the six powers, but also the most backward. The country was almost entirely agricultural, although it had begun to develop some industries. It was worried about the growing power of Germany, and had a long history of rivalry with Austria-Hungary.
In 1917, a revolution in Russia brought in a Communist government. The Bolshevik Party had to fight a civil war in Russia and did not want to fight the Germans. They made peace with Germany in February 1918, with the result that the Germans could now transfer hundreds of thousands of troops back to the Western Front.