On 23 August 1939 the Soviet Union asserted its control over the Baltic states with the Molotov–Ribbentrop Pact. 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. After four days of negotiations, the Estonians had no choice but to accept naval, air and army bases. Soviet troop numbers in Estonia were put at 25,000. The mutual assistance treaty was signed on 28 September. As a result, soon two other Baltic states yielded to Soviet pressure.