How did the German-Soviet Nonaggression Pact affect Estonia in 1939?

The German-Soviet Nonaggression Pact of August 23, 1939, fundamentally altered Estonia's sovereignty by including a secret protocol that assigned the country to the Soviet sphere of influence.1

This agreement paved the way for the following sequence of events:

• Mutual Assistance Treaty: On September 28, 1939, the Soviet Union forced Estonia to sign a treaty of mutual assistance, which granted the Soviets the right to establish and occupy military bases within the country.2
• Occupation and Annexation: In June 1940, following a Soviet ultimatum, military forces occupied the entire nation. By August 1940, Estonia was formally incorporated into the U.S.S.R. as a constituent republic, a process that involved the arrest and deportation of many Estonian political leaders.2
• Repression: During the initial year of Soviet occupation, tens of thousands of Estonians were killed or deported.2

For decades, the Soviet Union denied the existence of the secret protocols, as they served as evidence of the involuntary annexation of the Baltic states.1