The secret protocols to the Nazi–Soviet Pact signed by the USSR and Germany on 23 August and 28 September 1939 predetermined the fate of the Lithuanian state. By 10 October 1939 the republic of Lithuania had been pressurised into signing a mutual assistance treaty with the USSR. This was not entirely without advantage to Lithuania since part of the Vilnius region was restored to it by the Soviets after having been part of Poland since 1921. However, the treaty deprived Lithuania of the status of a neutral state. Compelled to consent to the stationing of 20,000 Soviet troops on its territory, Lithuania became dependent on the Soviet Union. That was the first stage in the annexation of Lithuania.

Under the terms of the treaty the Soviet Government guaranteed not to interfere in the internal affairs of Lithuania. Nevertheless, in the early summer of 1940, the Soviets made several groundless accusations against the Lithuanian Government. They alleged that Lithuania had formed a military alliance of the Baltic states directed against the Soviet Union, thus violating the mutual assistance treaty and, furthermore, had been involved in kidnapping Red Army soldiers. Taking advantage of the international situation the Soviets advanced one more step towards full incorporation of Lithuania into the USSR on 14 June 1940.

How did the relations between Lithuania and the USSR develop in May and June 1940? In attempting to answer this question, reference will be made to data from the Lithuanian archives, published in the works of G. Rudis and V. Zalys, and to the press reports of the time.