Complex sentence: In 1935, Müller decided to end his apprenticeship to join the Heer, just after his twenty-first birthday in October, he was posted to the 1st Company, 1st Pioneer (Engineer) Battalion stationed in Königsberg and received his basic training.
Split into simpler sentences: In 1935, Müller decided to end his apprenticeship to join the Heer. Just after his twenty-first birthday in October, he was posted to the 1st Company, 1st Pioneer (Engineer) Battalion stationed in Königsberg and received his basic training.