When Nazi Germany invaded Poland in September of 1939, Mikołajczyk was a private in the Polish army, and served in the defence of Warsaw. After the fall of Warsaw he escaped to Hungary, where he was interned. He soon escaped and made his way to Paris via Yugoslavia and Italy. By the end of November, Mikołajczyk had reached France where he was immediately asked to join the Polish government in exile as deputy Chairman of the Polish National Council. In 1941 he was appointed Minister of the Interior and became Prime Minister Władysław Sikorski's Deputy Prime Minister. In April 1943 the Germans had announced that they had discovered the graves of almost 22,000 Polish officers who had been murdered by the Soviets at Katyń Wood. The Soviet government said that the Germans had fabricated the discovery. The Allied governments, for diplomatic reasons, formally accepted this, but Mikołajczyk's government refused to do so, and Stalin then severed relations with the government in exile.