In March 1939, Wilcke volunteered for service with the Condor Legion during the Spanish Civil War. For a few weeks, he flew with 1. "Staffel" (1st squadron) of "Jagdgruppe" 88 (J/88—88th Fighter Group) without claiming any aerial victories. He was awarded the Spanish Cross in Bronze with Swords () for his service in Spain. In Spain he became friends with Werner Mölders and when Mölders was appointed "Gruppenkommandeur" (group commander) of the newly created III. "Gruppe" of JG 53, he selected Wilcke as "Staffelkapitän" (squadron leader) of the 7. "Staffel" (7th squadron) of JG 53.