Plans to acquire Spitfires and Lysanders from Britain were thwarted when the outbreak of World War II in 1939 forced Britain to cancel all export orders. After the defeat of Poland, Estonia was forced to accept a Mutual Assistance Pact with the Soviet Union, signed on 28 September 1939. This allowed the Russians to establish military bases in Estonia, which were later used in the Winter War against Finland. On 17 June 1940 the three Baltic States were invaded by Soviet forces. During the June 1940 invasion the Air Defence took no action and subsequently aircraft remained locked in their hangars. The air force became the Aircraft Squadron of the 22nd Territorial Corps of the Soviet Army in the summer of 1940.