At a time of increasing cooperation between the Western Allies and the Soviet Union in the wake of the Nazi invasion of 1941 , the influence of the Polish government-in-exile was seriously diminished by the death of Prime Minister Władysław Sikorski , its most capable leader , in a plane crash on 4 July 1943 . Around that time , Polish-communist civilian and military organizations opposed to the government , led by Wanda Wasilewska and supported by Stalin , were formed in the Soviet Union
plane crash on 4 July 1943
1943 Gibraltar B-24 crash
History of Poland