On 4 July 1943 the Polish Prime Minister-in-Exile, General Wladyslaw Sikorski, who was also Commander-in-Chief of the Polish Armed Forces in the West, died in an air crash off Gibraltar as he was returning to England from an inspection tour of Polish forces in the Mediterranean theatre. Until the Germans' April 1943 discovery of mass graves of 28,000 executed Polish military reserve officers at Katyn, near Smolensk in Russia, Sikorski had wished to work with the Soviets. After Hitler's invasion of the Soviet Union in June 1941, the Soviets' importance to the Western alliance had grown while British support for Polish aspirations had begun to decline. As the war progressed, Polish plans to more completely incorporate Poland's underground Home Army into the broader strategy of the Western allies—including contingency plans to move Polish Air Force fighter squadrons, and the Polish Parachute Brigade, to Poland—foundered on British and American reluctance to antagonise a vital Soviet ally hostile to Polish autonomy; on the distance from British-controlled bases to occupied Poland, which lay at the extreme flying range of available aircraft; and on the frittering away of the Polish Parachute Brigade on a patently flawed British operation at Arnhem, the Netherlands.