Polish–Lithuanian border of 1918–1920 as permanent nor did they ever acknowledge the sovereignty of the puppet Republic of Central Lithuania. In 1922 the Republic of Central Lithuania voted to join Poland and the choice was later accepted by the League of Nations, The area granted to Lithuania by the Bolsheviks in 1920 continued to be claimed by Lithuania, with the city of Vilnius being treated as that state's official capital and the temporary capital in Kaunas, and the states officially remained at war. It was not until the Polish ultimatum of 1938, that the two states resolved diplomatic relations. Some historians speculated, that the loss of Vilnius might have nonetheless safeguarded the very existence of the Lithuanian state in the interwar period. Despite an alliance with the Soviets (Soviet–Lithuanian Peace Treaty) and the war with Poland, Lithuania was very close to being invaded by the Soviets in the summer of 1920 and having been forcibly converted into a socialist republic. They believe it was only the Polish victory against the Soviets in the Polish–Soviet War (and the fact that the Poles did not object to some form of Lithuanian independence) that derailed the Soviet plans and gave Lithuania an experience of interwar independence. In 1939, the Soviets proposed to sign the Soviet–Lithuanian Mutual Assistance Treaty. According to this treaty, about one-fifth of the Vilnius Region, including the city of Vilnius itself, was returned to Lithuania in exchange for stationing 20,000 Soviet troops in Lithuania. Lithuanians at first did not want to accept this, but later the Soviet Union said that troops would enter Lithuania, anyway, so Lithuania accepted the deal. 1/5 of the Vilnius region was ceded, even though the Soviet Union always recognised the whole Vilnius region as part of Lithuania previously. The Soviet Union was awarded the Vilnius region during the Yalta Conference, and it subsequently became part of the Lithuanian SSR. About 150,000 of the Polish population was repatriated from Lithuanian SSR to Poland. Ethnography The area was originally inhabited by Lithuanian Balts. It was subjected to East Slavic and Polish cultural influences and settlement, which led to its gradual Ruthenization and Polonization. According to Norman Davies, Vilnius was culturally Polish by the 17th century. By the 18th and 19th centuries, the city was almost completely surrounded by Slavs, while the Vilnius region became exceptionally ethnically diverse Belarusian-Polish-Lithuanian territory. The Belarusian population moved into the areas devastated