In the history of Lithuania, the Vilnius Conference (Vilniaus konferencija) or Vilnius National Conference met between September 18, 1917, and September 22, 1917, and began the process of establishing a Lithuanian state based on ethnic identity and language that would be independent of the Russian Empire, Poland, and the German Empire. It elected a twenty-member Council of Lithuania that was entrusted with the mission of declaring and re-establishing an independent Lithuania. The Conference, hoping to express the will of the Lithuanian people, gave legal authority to the council and its decisions. While the Conference laid the basic guiding principles of Lithuanian independence, it deferred any matters of the political structure of future Lithuania to the Constituent Assembly, which would later be elected in a democratic manner. Lithuania existed as an independent state from the beginning of the 13th century until 1569, when it entered into a union with Poland, forming the Polish–Lithuanian Commonwealth. The Commonwealth ceased to exist after the Partitions of the Polish–Lithuanian Commonwealth in the late 18th century. Most of the Lithuanian territory was incorporated into the Russian Empire. A Lithuanian independence movement arose during the 19th century, based on concepts of national self-determination that were formalized in Woodrow Wilson's Fourteen Points speech in January 1918. During the course of World War I, the German Army invaded Russia and soon entered the territory which comprised Lithuania. In 1915, the Germans assumed control and organized a military administration known as Ober Ost (short for der Oberbefehlshaber der gesamten deutschen Streitkräfte im Osten: "supreme command of all German forces in the East"). At first, the Germans simply exploited Lithuania for the benefit of their war effort. As the war progressed, it became evident that the two-front war that Germany was engaged in would necessitate a compromise peace with the Russian Empire. This necessitated a re-thinking of strategies concerning the occupied territories in the east.