コンドル軍団
On 26 August 1937, the Condor Legion attacked ships in the harbor of Gijón. Flying fighter escort to the bombers, Seiler claimed his first aerial victory when he shot down a Polikarpov I-15 fighter. He claimed his second aerial victory on 4 September, a Polikarpov I-16 shot down over Asturias. On 30 October, command of the Condor Legion transferred from "Generalmajor" Hugo Sperrle to "Generalmajor" Hellmuth Volkmann. Volkmann reorganized J/88, placing 2. "Staffel" under the command of "Oberleutnant" (First Lieutenant) Joachim Schlichting. Seiler claimed his third aerial victory on 29 November.