Short answers, pulled from the story.
The Molotov, Ribbentrop Pact was signed on the 23rd of August 1939. This neutrality pact between Nazi Germany and the Soviet Union included a secret additional protocol that partitioned Eastern Europe into German and Soviet spheres of influence.
Germany invaded Poland on the 1st of September 1939, followed by the Red Army invading on the 17th of September to occupy territory assigned to it by the pact. The agreement stipulated that areas east of the Pisa, Narew, Vistula, and San rivers would go to the Soviet Union while Germany occupied the west.
Soviet NKVD troops raided border posts in Lithuania Estonia and Latvia in mid-June 1940. State administrations were liquidated and replaced by Soviet cadres who deported or killed hundreds of thousands of citizens from these nations before their admission into the Soviet Union.
Germany and the Soviet Union entered an intricate trade pact on the 11th of February 1940 that was over four times larger than the one signed in August 1939. This agreement provided Germany with millions of tons of cereals oil cotton phosphates and soybeans while supplying the Soviets with naval cruisers warplanes and heavy machinery.
The secret protocols were first published on the 22nd of May 1946 in a front-page story by the St. Louis Post-Dispatch after being recovered from microfilm records found by US State Department employee Wendell B. Blancke. The documents had been buried by Karl von Loesch during the war and later transferred to Allied forces for examination.