Following his arrival in Spain, Balthasar served with Kampfgruppe K/88 and Aufklärungsgruppe A/88 flying bomber and reconnaissance missions in Junkers Ju 52 and Heinkel He 70.
In September 1937, Wilhelm Balthasar joined Jagdgruppe 88 J/88 and claimed six more victories (including four Tupolev SB bombers in one mission on 7 February 1938) flying He-51 and the legendary Messerschmitt Bf 109.