In World War II, he had to serve in the army in the Invasion of Poland. He wrote in a letter to his brother stating "Es gibt nichts, was einen Krieg rechtfertigt, und es ist jedes Mittel erlaubt, das einen Krieg verhindert," "i.e.", that nothing justifies war, and all means are permitted to prevent a war. Back at Breslau, he was ready to take the position of Ober-Präsident of Silesia in the in a democratic new beginning after a successful revolution. After the assassination attempt on Hitler on 20 July 1944 failed, Leuninger was arrested on 26 September, accused of not having reported the peace negotiations of which he was aware. This was a crime of high treason ("Hochverrat") under the contemporary penal code. He remained in custody for several months, while his wife escaped to the West, and their sons were enlisted. He was tried by the People's Court on 28 February 1945. He was sentenced to death, and executed the following day on 1 March 1945 in Plötzensee Prison in Berlin, at the age of 46. In a sermon in an ecumenical memorial service at the site on 20 July 1999, Karl Meyer said that Leuninger walked to his execution singing psalms.