Complex sentence: After the end of the War, he served on the eastern border in Upper Silesia as part of the free corps, after his promotion to captain, he left the army in September 1920, to be a lieutenant of the Prussian police unit.
Split into simpler sentences: After the end of the War, he served on the eastern border in Upper Silesia as part of the free corps. After his promotion to captain, he left the army in September 1920 to be a lieutenant of the Prussian police unit.