At the onset of World War I, Jung voluntarily joined the imperial armies and reached the rank of lieutenant. After the end of the war, he participated in the suppression of the Bavarian Soviet Republic in the spring of 1919 and in the resistance against the French occupation of the Palatinate, during which he participated in the assassination of Franz Josef Heinz. Expelled by the French authorities, Jung moved to Munich, where, in 1925, he opened a law firm and dampened his political activism slightly.