Following the capitulation of Germany in November 1918, the first Provisional Constitution of Lithuania was adopted and the first government of Prime Minister Augustinas Voldemaras was organized. At the same time, the army and other state institutions began to be organized. Lithuania fought three wars of independence: against the Bolsheviks who proclaimed the Lithuanian Soviet Socialist Republic, against the Bermontians, and against Poland. As a result of the staged Żeligowski’s Mutiny in October 1920, Poland took control of Vilnius Region and annexed it as Wilno Voivodeship in 1922. Lithuania continued to claim Vilnius as its de jure capital (the de facto, provisional capital being Kaunas) and relations with Poland remained particularly tense and hostile for the entire interwar period. In January 1923, Lithuania staged the Klaipėda Revolt and captured Klaipėda Region (Memel territory) which was detached from East Prussia by the Treaty of Versailles. The region became an autonomous region of Lithuania.
On 15 May 1920, the first meeting of the democratically elected constituent assembly took place. The documents it adopted, i. e. the temporary (1920) and permanent (1922) constitutions of Lithuania, strove to regulate the life of the new state. Land, finance, and educational reforms started to be implemented. The currency of Lithuania, the Lithuanian litas, was introduced. The University of Lithuania was opened. All major public institutions had been established. As Lithuania began to gain stability, foreign countries started to recognize it. In 1921 Lithuania was admitted to the League of Nations.
On 17 December 1926, a military coup d’état took place, resulting in the replacement of the democratically elected government with a conservative authoritarian government led by Antanas Smetona. Augustinas Voldemaras was appointed to form a government. The so-called authoritarian phase had begun strengthening the influence of one party, the Lithuanian Nationalist Union, in the country. In 1927, the Seimas was dissolved. A new constitution was adopted in 1928, which consolidated presidential powers. Gradually, opposition parties were banned, censorship was tightened, and the rights of national minorities were narrowed.
On 20 March 1939, after years of rising tensions, Lithuania was handed an ultimatum by Nazi Germany demanding it relinquish the Klaipėda Region. Two days later, the Lithuanian government accepted the ultimatum. When Nazi Germany and Soviet Union concluded the Molotov–Ribbentrop Pact, Lithuania was initially assigned to the German sphere of influence but was later transferred to the Soviet sphere. At the outbreak of World War II, Lithuania declared neutrality.
In October 1939, Lithuania was forced to sign the Soviet–Lithuanian Mutual Assistance Treaty: five Soviet military bases with 20,000 troops were established in Lithuania in exchange for Vilnius, which the Soviets had captured from Poland. Delayed by the Winter War with Finland, the Soviets issued an ultimatum to Lithuania on 14 June 1940. They demanded the replacement of the Lithuanian government and that the Red Army be allowed into the country. The government decided that, with Soviet bases already in Lithuania, armed resistance was impossible and accepted the ultimatum. President Smetona left the country, hoping to form a government in exile, while more than 200,000 Soviet Red Army soldiers crossed the Belarus–Lithuania border. The next day, identical ultimatums were presented to Latvia and Estonia. The Baltic states were occupied. The Soviets followed semi-constitutional procedures for transforming the independent countries into soviet republics and incorporating them into the Soviet Union.