Mueller, an opponent of Adolf Hitler, was recruited to the German resistance by Hans Oster. A Catholic, Mueller made several visits to Rome where he met Pius XII. Mueller's attempts to get help from the Pope ended in failure.
The Gestapo grew suspicious of Mueller and he was arrested on 5th April, 1943. After enduring 200 interrogations, without revealing anything, he was sent to Buchenwald. Later he was moved to Dachau. Although experiencing nearly two years of starvation rations and being led to the gallows twice, Mueller was still alive when liberated by the Allies in April, 1945.
After the war Mueller became minister of justice in Bavaria. Josef Mueller died in 1979.