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August 23, 1939
The Soviet Union (USSR) and National Socialist Germany conclude a non-aggression treaty (the Hitler–Stalin Pact). Its secret protocol provides for the division of Poland and the inclusion of Finland, Estonia, Latvia, and Bukovina in the "sphere of interest" of the USSR.
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September 1, 1939
Germany attacks Poland, the Second World War begins. Declaration on neutrality adopted by Latvia.
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September 17, 1939
The troops of the USSR invade Poland.
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September 28, 1939
Germany and the USSR sign a border and friendship treaty, which divides Poland, includes Lithuania in the "sphere of interests" of the USSR and in secret protocols, provides for the exodus of ethnic Germans from the "sphere of interest" of the USSR, and cooperation of the secret services.
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October 5, 1939
Under military and political pressure, Latvia signs a Mutual Assistance Pact with the USSR, allowing the establishment of bases in western Latvia for 25,000 USSR soldiers of the Red Army and Soviet Navy.
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October 30, 1939
The Government of Latvia concludes an agreement with National Socialist Germany on the transfer of ethnic German Latvian citizens to Germany.
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June 12, 1940
The Soviet Navy begins a naval blockade of the Baltic States. At dawn on June 15, Soviet NKVD forces attack three posts of the Eastern Border Guards of Latvia. Buildings are burned, five persons killed, 37 captured and taken to Russia.
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June 16, 1940
The USSR submits an ultimatum to Latvia, demanding the resignation of the Latvian Cabinet of Ministers and unrestricted admission of USSR troops in Latvia. The Latvian government accepts the ultimatum without protest or resistance.