On 23 August 1939 Germany and the Soviet Union signed the Molotov–Ribbentrop Pact. The pact's secret protocol divided Poland, Lithuania, Latvia, Estonia and Finland into spheres of influence, with Estonia belonging to the Soviet sphere. On 24 September, the Soviet Union presented an ultimatum, demanding that Estonia sign a treaty of mutual assistance which would allow Soviet military bases into the country. The Estonian government felt that it had no choice but to comply, and the treaty was signed on 28 September. In May 1940, Red Army forces in bases were set in combat readiness and, on 14 June, the Soviet Union instituted a full naval and air blockade on Estonia. On the same day, the airliner Kaleva was shot down by the Soviet Air Force. On 16 June, USSR presented an ultimatum demanding completely free passage of the Red Army into Estonia and the establishment of a pro-Soviet government. Feeling that resistance was hopeless, the Estonian government complied and, on the next day, the whole country was occupied. On 6 August 1940, Estonia was annexed by the Soviet Union as the Estonian SSR.