Question: When did the country Grobina Castle is located become part of the soviet Union?

full_context: Document 1: Piltene Castle: Piltene Castle is a Bishopric of Courland castle in the town of Piltene in the historical region of Courland, in western Latvia. Until the 16th century it served as a capital of Bishopric of Courland.
Document 2: Estonia: On 6 August 1940, Estonia was annexed by the Soviet Union as the Estonian SSR. The provisions in the Estonian constitution requiring a popular referendum to decide on joining a supra-national body were ignored. Instead the vote to join the Soviet Union was taken by those elected in the elections held the previous month. Additionally those who had failed to do their "political duty" of voting Estonia into the USSR, specifically those who had failed to have their passports stamped for voting, were condemned to death by Soviet tribunals. The repressions followed with the mass deportations carried out by the Soviets in Estonia on 14 June 1941. Many of the country's political and intellectual leaders were killed or deported to remote areas of the USSR by the Soviet authorities in 1940–1941. Repressive actions were also taken against thousands of ordinary people.
Document 3: Koberg Castle: Koberg Castle is a castle in Västergötland, Sweden. It is located near the lake Vanderydsvattnet, about 10 km south of Trollhättan. It is the residence of Princess Désirée, Baroness Silfverschiöld and her husband Baron Nicklas Silfverschiöld.
Document 4: Rosenburg: Rosenburg is a castle in the municipality Rosenburg-Mold, Lower Austria, Austria. Rosenburg is on a cliff above the valley of the River Kamp at an elevation of above sea level. It is one of Austria's most visited Renaissance castles. It is situated in the middle of a nature reserve - the Naturpark Kamptal - which adds to its appeal. There are various castles and fortifications in Austria and Germany that bear the name "Rosenburg", but if people use the term without further specifications, it may be understood that they refer to this site in Lower Austria. The well-known Austrian folk song "Es liegt ein Schloss in Österreich" ("There is a castle in Austria") is often assumed to refer to the Rosenburg.
Document 5: Lithuanian Soviet Socialist Republic: The Lithuanian Soviet Socialist Republic (Lithuanian SSR; Lithuanian: Lietuvos Tarybų Socialistinė Respublika; Russian: Литовская Советская Социалистическая Республика, Litovskaya Sovetskaya Sotsialisticheskaya Respublika), also known as Soviet Lithuania or Lithuania was a republic of the Soviet Union. It existed from 1940 to 1990.
Document 6: Dissolution of the Soviet Union: On June 14, 1987, about 5,000 people gathered again at Freedom Monument in Riga, and laid flowers to commemorate the anniversary of Stalin's mass deportation of Latvians in 1941. This was the first large demonstration in the Baltic republics to commemorate the anniversary of an event contrary to official Soviet history. The authorities did not crack down on demonstrators, which encouraged more and larger demonstrations throughout the Baltic States. The next major anniversary after the August 23 Molotov Pact demonstration was on November 18, the date of Latvia’s independence in 1918. On November 18, 1987, hundreds of police and civilian militiamen cordoned off the central square to prevent any demonstration at Freedom Monument, but thousands lined the streets of Riga in silent protest regardless.
Document 7: Grobiņa Castle: Grobiņa Castle is a medieval castle located in the town of Grobiņa, Latvia, in western Courland. The ancient Curonian castle hill ("Skābāržu kalns") is located only 100 m from the castle. It is supposed to be the famous Seeburg, which is mentioned in Scandinavian sources as early as the 9th century.
Document 8: Rēzekne Castle: Rēzekne Castle ruins are located in the centre of Rēzekne, Latvia, a city in the centre of Latgale. The castle served as a base of the local Livonian order landlords until the 16th century and also as the main military support base for battles against Russians and Lithuanians. Today, fragments of the stone walls and the foundation can be seen on the ancient castle hill.
Document 9: Latvian Soviet Socialist Republic: Following the Welles Declaration of July 23, 1940, the annexation of Latvia into the Soviet Union (USSR) on 5 August 1940 was not recognized as legitimate by the United States, the European Community, and recognition of it as the nominal fifteenth constituent republic of the USSR was withheld for five decades. Its territory was subsequently conquered by Nazi Germany in June -- July 1941, before being retaken by the Soviets in 1944 -- 1945. Nevertheless, Latvia continued to exist as a de jure independent country with a number of countries continued to recognize Latvian diplomats and consuls who still functioned in the name of their former governments.
Document 10: Skelbo Castle: Skelbo Castle is a ruined 14th century keep, located on the high shore at the mouth of Loch Fleet in the Highlands in Scotland. The remaining wall is best preserved at the northern side of the castle. The remains are protected as a scheduled monument.
Document 11: Molotov–Ribbentrop Pact: In mid-June 1940, when international attention was focused on the German invasion of France, Soviet NKVD troops raided border posts in Lithuania, Estonia and Latvia. State administrations were liquidated and replaced by Soviet cadres, in which 34,250 Latvians, 75,000 Lithuanians and almost 60,000 Estonians were deported or killed. Elections were held with single pro-Soviet candidates listed for many positions, with resulting peoples assemblies immediately requesting admission into the USSR, which was granted by the Soviet Union. The USSR annexed the whole of Lithuania, including the Scheschupe area, which was to be given to Germany.
Document 12: Post-Soviet states: The post-Soviet states, also collectively known as the former Soviet Union (FSU) or former Soviet Republics, are the 15 independent states that emerged from the Union of Soviet Socialist Republics in its dissolution in December 1991, with Russia internationally recognised as the successor state to the Soviet Union. On March 11, 1990, Lithuania was the first to declare its independence, with Estonia and Latvia following suit in August 1991. All three Baltic states claimed continuity from the original states that existed prior to their annexation by the Soviet Union in 1944 and were admitted to the United Nations on 17 September 1991. The remaining 12 republics all subsequently seceded. 12 of the 15 states, excluding the Baltic states, initially formed the CIS and most joined CSTO, while the Baltic states focused on European Union and NATO membership.
Document 13: Raseborg Castle: The first written data about the castle is from 1378. Its main purpose was to protect Sweden's interests in southern Finland against the Hanseatic city of Tallinn. The castle was originally built on a small island in the north end of a sea bay. The historians think that the castle was built in 3 different stages over time from the 14th to the 16th century.
Document 14: History of Latvia: The First Latvian National Awakening began in the 1850s and continued to bear fruit after World War I when, after two years of struggle in the Latvian War of Independence, Latvia finally won sovereign independence, as recognised by Soviet Russia in 1920 and by the international community in 1921. The Constitution of Latvia was adopted in 1922. Political instability and effects of the Great Depression led to the May 15, 1934 coup d'état by Prime Minister Kārlis Ulmanis. Latvia's independence was interrupted in June -- July 1940, when the country was occupied and incorporated into the Soviet Union. In 1941 it was invaded and occupied by Nazi Germany, then reconquered by the Soviets in 1944 -- 45.
Document 15: Estonia: In 1989, during the "Singing Revolution", in a landmark demonstration for more independence, more than two million people formed a human chain stretching through Lithuania, Latvia and Estonia, called the Baltic Way. All three nations had similar experiences of occupation and similar aspirations for regaining independence. The Estonian Sovereignty Declaration was issued on 16 November 1988. On 20 August 1991, Estonia declared formal independence during the Soviet military coup attempt in Moscow, reconstituting the pre-1940 state. The Soviet Union recognised the independence of Estonia on 6 September 1991. The first country to diplomatically recognise Estonia's reclaimed independence was Iceland. The last units of the Russian army left on 31 August 1994.
ground_truth: 5 August 1940
sub_queries: Grobiņa Castle >> country Answer: Latvia
when did Latvia become part of the soviet union Answer: 5 August 1940