In December 1938 Stingl was conscripted as a "Fahnenjunker" ("loosely, "trainee officer""), working on operations. During the Second World War he served in the Luftwaffe ("German air force"), becoming a spotter, a pilot and, by the time of his capture in 1945, an Oberleutnant (senior lieutenant officer). He ended the war with more than 200 missions flown. Sources keen to reconcile his military service on behalf of Hitler's Germany with the increasingly horrendous atrocities for which the regime was responsible stress that for Stingl, as a good catholic boy, military service to the fatherland would have been an obligation of honour even if he was critical of the regime. The pacifist's concept of a "duty to desert" (widely followed by sympathizers with the - since 1933 illegal - Communist Party) would have been unthinkable. By the time war ended Stingl was being held by the English in Schleswig-Holstein as a prisoner of war. He was released relatively soon however. By the end of 1945 he had been forced by the ethnic cleansing of those times to leave his home region: with his young family he relocated to Berlin.