Short answers, pulled from the story.
The Soviet Union first occupied Estonia, Latvia, and Lithuania in June 1940, following ultimata issued to each government. Lithuania accepted Soviet entry on the 15th of June 1940, with Latvia and Estonia following on the 16th of June. All three were formally incorporated into the USSR by August 1940.
The Molotov-Ribbentrop Pact was a ten-year non-aggression agreement signed by the Soviet Union and Nazi Germany in the early hours of the 24th of August 1939. A secret protocol divided northern and eastern Europe into spheres of influence, assigning Finland, Estonia, and Latvia to the Soviet sphere. A second secret protocol in September 1939 also assigned most of Lithuania to the Soviet Union, providing the basis for the 1940 occupation.
Nazi Germany occupied the Baltic states beginning in July 1941 after invading the Soviet Union on the 22nd of June 1941. Germany incorporated the territories into the Reichskommissariat Ostland, governed by Hinrich Lohse. The German occupation lasted until the Soviet Baltic Offensive, which launched on the 14th of September 1944 and recaptured most of the region, with remaining German forces trapped in the Courland Pocket until capitulating on the 10th of May 1945.
The total number deported from the Baltic states between 1944 and 1955 has been estimated at over half a million people: 124,000 from Estonia, 136,000 from Latvia, and 245,000 from Lithuania. In March 1949 alone, Soviet authorities organized a single mass deportation of 90,000 Baltic nationals. The estimated death toll among Lithuanian deportees between 1945 and 1958 was 20,000, including 5,000 children.
Lithuania declared independence on the 11th of March 1990, and Estonia and Latvia made their declarations in the summer of 1990. The Soviet government formally recognized the independence of all three Baltic states on the 6th of September 1991, following the failed August coup against Mikhail Gorbachev. The last Russian soldier left Baltic soil in October 1999, when the Skrunda-1 radar site was returned to Latvia.
The Baltic Way was a human chain spanning all three Baltic states that took place on the 23rd of August 1989, the fiftieth anniversary of the Molotov-Ribbentrop Pact. It was the largest organized demonstration against Soviet rule across the three countries. That December, the Congress of People's Deputies of the Soviet Union condemned the Molotov-Ribbentrop Pact and its secret protocol as "legally untenable and invalid."