The Baltic states, the United States and its courts of law, the European Parliament, the European Court of Human Rights and the United Nations Human Rights Council have all stated that these three countries were invaded, occupied and illegally incorporated into the Soviet Union under provisions of the 1939 Molotov–Ribbentrop Pact, first by the Soviet Union, then by Nazi Germany from 1941 to 1944, and again by the Soviet Union from 1944 to 1991. This policy of non-recognition has given rise to the principle of legal continuity, which holds that de jure, or as a matter of law, the Baltic states had remained independent states under illegal occupation throughout the period from 1940 to 1991.
In its reassessment of Soviet history that began during perestroika in 1989, the USSR condemned the 1939 secret protocol between Germany and itself. However, the USSR never formally acknowledged its presence in the Baltics as an occupation or that it annexed these states, and considered the Estonian, Latvian and Lithuanian Soviet Socialist Republics as its constituent republics. Nationalist-patriotic Russian historiography and school textbooks continue to maintain that the Baltic states voluntarily joined the Soviet Union after their peoples all carried out socialist revolutions independent of Soviet influence. The Russian government and its state officials insist that incorporation of the Baltic states was in accordance with international law and gained de jure recognition by the agreements made in the Yalta and Potsdam conferences and by the Helsinki Accords, whereas the Accords only committed existing frontiers would not be violated. However, Russia agreed to Europe's demand to "assist persons deported from the occupied Baltic states" upon joining the Council of Europe. Additionally, when the Russian Soviet Federative Socialist Republic signed a separate treaty with Lithuania in 1991, it acknowledged that the 1940 annexation was a violation of Lithuanian sovereignty and recognised the de jure continuity of the Lithuanian state.
Most Western governments maintained that Baltic sovereignty had not been legitimately overridden and thus continued to recognise the Baltic states as sovereign political entities represented by the legations appointed by the pre-1940 Baltic states which functioned in Washington and elsewhere. De facto independence was restored to the Baltic states in 1991 during the dissolution of the Soviet Union. Russia started to withdraw its troops from the Baltics (starting from Lithuania) in August 1993. The full withdrawal of troops deployed by Moscow was completed in August 1994. Russia officially ended its military presence in the Baltics in August 1998 by decommissioning the Skrunda-1 radar station in Latvia. The dismantled installations were repatriated to Russia and the site returned to Latvian control, with the last Russian soldier leaving Baltic soil in October 1999.
Early in the morning of August 24, 1939, the Soviet Union and Germany signed a ten-year non-aggression pact, called the Molotov–Ribbentrop pact. The pact contained a secret protocol by which the states of Northern and Eastern Europe were divided into German and Soviet "spheres of influence". In the north, Finland, Estonia and Latvia were assigned to the Soviet sphere. Poland was to be partitioned in the event of its "political rearrangement"—the areas east of the Narev, Vistula and San Rivers going to the Soviet Union while Germany would occupy the west. Lithuania, adjacent to East Prussia, would be in the German sphere of influence, although a second secret protocol agreed in September 1939 assigned the majority of Lithuanian territory to the Soviet Union. According to this secret protocol, Lithuania would regain its historical capital Vilnius, previously subjugated during the inter-war period by Poland.
Following the end of Soviet invasion of Poland on 6 October, the Soviets pressured Finland and the Baltic states to conclude mutual assistance treaties. The Soviets questioned the neutrality of Estonia after the escape of an interned Polish submarine on 18 September. A week later on 24 September, the Estonian foreign minister was given an ultimatum in Moscow. The Soviets demanded the conclusion of a treaty of mutual assistance to establish military bases in Estonia. The Estonians had no choice but to accept naval, air and army bases on two Estonian islands and at the port of Paldiski. The corresponding agreement was signed on 28 September 1939. Latvia followed on 5 October 1939 and Lithuania shortly thereafter, on 10 October 1939. The agreements permitted the Soviet Union to establish military bases on the Baltic states' territory for the duration of the European war and to station 25,000 Soviet soldiers in Estonia, 30,000 in Latvia and 20,000 in Lithuania from October 1939.
In September and October 1939, the Soviet government compelled the Baltic states to conclude mutual assistance pacts which gave it the right to establish Soviet military bases. In May 1940, the Soviets turned to the idea of direct military intervention, but still intended to rule through puppet regimes. Their model was the Finnish Democratic Republic, a puppet regime set up by the Soviets on the first day of the Winter War. The Soviets organised a press campaign against the allegedly pro-Allied sympathies of the Baltic governments. In May 1940, the Germans invaded France, which was overrun and occupied a month later. In late May and early June 1940, the Baltic states were accused of military collaboration against the Soviet Union by holding meetings the previous winter.:43 On 15 June 1940, the Lithuanian government had no choice but to agree to the Soviet ultimatum and permit the entry of an unspecified number of Soviet troops. President Antanas Smetona proposed armed resistance to the Soviets but the government refused, proposing their own candidate to lead the regime. However, the Soviets refused this offer and sent Vladimir Dekanozov to take charge of affairs while the Red Army occupied the state.
On 16 June 1940, Latvia and Estonia also received ultimata. The Red Army occupied the two remaining Baltic states shortly thereafter. The Soviets dispatched Andrey Vyshinsky to oversee the takeover of Latvia and Andrei Zhdanov to oversee the takeover of Estonia. On 18 and 21 June 1940, new "popular front" governments were formed in each Baltic country, made up of Communists and fellow travelers. Under Soviet surveillance, the new governments arranged rigged elections for new "people's assemblies." Voters were presented with a single list, and no opposition movements were allowed to file and to get the required turnout to 99.6% votes were forged.:46 A month later, the new assemblies met, with their sole item of business being resolutions to join the Soviet Union. In each case, the resolutions passed by acclamation. The Supreme Soviet of the Soviet Union duly accepted the requests in August, thus giving legal sanction to the takeover. Lithuania was incorporated into the Soviet Union on 3 August, Latvia on 5 August, and Estonia on 6 August 1940. The deposed presidents of Estonia (Konstantin Päts) and Latvia (Kārlis Ulmanis) were imprisoned and deported to the USSR and died later in the Tver region and Central Asia respectively. In June 1941, the new Soviet governments carried out mass deportations of "enemies of the people". It is estimated that Estonia alone lost 60,000 citizens.:48 Consequently, many Balts initially greeted the Germans as liberators when they invaded a week later.