World War 2 was the greatest war in the history of mankind. However, we are going to only talk about the part that is related to the Baltic states - Molotov - Ribbentrop Pact(MRP) and the battles and wars following it.
In 1939, Nazi Germany and the Soviet Union signed the MRP dividing Eastern Europe. After that, in late June Bessarabia, North Bukovina and Hertsa were annexed into the Soviet Union, in July puppet states were created in the Baltics, in August, the Baltic states were annexed into the Soviet Union, in September Germany and Soviets divided Poland, and in November the Soviet Union invades Finland. In March 1940 the Winter War is over, and Soviet Union take over Karelia.
The Germans invaded the USSR in breach of a non-aggression pact during 1941. The Balts thought this was time to join against the communists, Jews, Roma and Russians. Groups like The Arajs Kommando and Estonian conscript auxiliary police aided and abetted the German Wehrmacht, SS and Einezatzgruppen in carrying out the Holocaust. The Finns were a German allies in the region, but opposed the Holocaust. Latvian Central Council (LCC) (Latvian: Latvijas Centrālās Padomes, LCP) openly opposed the Nazis and rebelled against the Soviet Partisan movement. Eventually, after much suffering, the Baltic people realised they were being used as cannon-fodder by the Germans, who were expelled by the Soviets in 1944. They then violently purged the Baltics and expelled the Germans from Kaliningrad Oblast, at that time East Prussia.
The war finally ends up with Poland of today, and the Baltic states as part of Russia.