Temporary capital of Lithuania during the invasion of Poland, to Lithuania according to the Soviet–Lithuanian Mutual Assistance Treaty. Since then Vilnius has been the capital of the Lithuanian SSR and then independent Lithuania. Lithuania refused to have diplomatic relations between Poland and Lithuania until the ultimatum of 1938. Railroad traffic, telegraph lines or even mail could not cross the Polish–Lithuanian border. Lithuania continued to declare Vilnius as its capital in all official documents, including the constitution. Since the city was controlled by Poland, all Lithuanian authorities were transferred to the city of Kaunas, which became the seat of the government. To reconcile reality with
Act of Independence of Lithuania the first Cabinet of Lithuania was formed, and the Council of Lithuania gained control over the territory of Lithuania. Independent Lithuania, although it would soon be battling the Wars of Independence, became a reality. The laconic Act is the legal basis for the existence of modern Lithuania, both during the interwar period and since 1990. The Act formulated the basic constitutional principles that were and still are followed by all Constitutions of Lithuania. The Act itself was a key element in the foundation of Lithuania's re-establishment of independence in 1990. Lithuania, breaking away from the Soviet Union, stressed that it