The Soviet Union, which invaded Poland on 17 September 1939, accused Estonia of conspiring with the Polish seamen and "aiding them to escape" and challenged the neutrality of Estonia. The Soviets demanded to be allowed to establish military bases on Estonian soil and threatened war if Estonia did not comply. That served as a political cover for their actions since the Molotov-Ribbentrop Pact had already given German approval to the Soviets' taking over the Baltic states. The "Orzeł" incident was used to force the "Pact of Defence and Mutual Assistance" on Estonia, which was signed on 28 September 1939 and allowed for the occupation and the annexation of Estonia by the Soviets in 1940.