On August 23, 1939 the Soviet Union asserted its control over the Baltic states with the Molotov–Ribbentrop Pact, which declared them as Soviet sphere of influence. On September 16, the Soviets and Japanese governments signed a cease-fire agreement. Next, the Soviets invaded Poland on 17 September, concluding operations on 6 October. After occupying eastern Poland, the Soviets pressured Finland and the Baltic states to conclude mutual assistance treaties. The Soviets questioned the neutrality of Estonia following the escape of a Polish submarine on 18 September. A week later, on 24 September, the Estonian foreign minister Karl Selter was given an ultimatum in Moscow. The Soviets demanded the conclusion of a treaty of mutual assistance which included the establishment of military bases in Estonia.