Taken from the Russia Today webpage (original source):
Since the 18th century all of what is now Latvia, Estonia and most of Lithuania were part of the Russian Empire. Before the end of the First World War, the Empire had collapsed and in 1918 Riga, Vilnius and Tallinn proclaimed their independence, but this independence was short-lived.
In 1940, after a pact between Stalin and Hitler, the Baltic States entered the Soviet Union. Nazi forces pushed the Soviets back in 1941 but the Red Army returned in 1944 to make the countries part of the USSR once again. Its independence was reestablished only with the collapse of the Soviet Union.
So, despite the fact that in relation to Russia, Latvia calls itself a republic with a 90 year history, there are essentially no documented facts to prove this.
As historian Vladimir Simindey points out, “Russia recognized Latvia’s independence after the collapse of the USSR just like it did with all the other former Soviet republics“.
Even whilst taking into account current affirmations that Latvia retained its independence throughout the years of Moscow rule, on paper it officially held the status of a Soviet republic.
Since the collapse of the USSR Latvia, Estonia and Lithuania went from Communism to NATO and EU membership. Now, they are keen to break away from their Soviet past. The Museum of Occupation in Riga makes little difference between Nazi occupation during WW2 and Latvia’s years within the USSR and grudges over history continue to poison Latvia’s relations with Moscow.