In 1775 Pirch began his career in the Prussian military, becoming a cadet in the Hessen-Kassel Nr. 45 infantry regiment. In 1787 he saw action in the Prussian invasion of Holland and in 1793 he participated in Siege of Mainz. He became a French prisoner for two years after Prussian defeat in Battle of Jena in 1806. When the War of the Sixth Coalition began Pirch was appointed commander of a brigade in April 1813. Serving under the Army of Bohemia, he fought in the battles of Lützen, Bautzen, Dresden, and Kulm, earning promotion to the rank of Major General and both classes of the Iron Cross. At the Battle of Leipzig he led the capture of the village of Probstheida, despite suffering heavy casualties in his brigade; for his role in the battle he was awarded the Pour le Mérite with oak leaves and the Russian Order of Saint Anna 1st class. In 1814 he participated in the Battle of Laon under the command of Blücher.