he commanded a Swiss regiment in Basel as Colonel. He then fought in the Valtellina War (1620–1622) and the Mantuan War of Succession (1628–1631). He reached the first highlight of his military career in 1633 with his appointment as Lieutenant General and secret war council of the Landgrave William V of Hesse-Kassel. William V was allied with the Swedes, so Peter Melander fought with the Hessian troops against the imperial army. In the Battle of Oldendorf on 28 June 1633, he commanded the center of the Protestant forces under Duke George of Brunswick-Calenberg and contributed much to the victory over the Imperial army and defeated them several more times as he chased them through Westphalia. He captured Hamm on 26 May 1634 and on 27 June 1634, he defeated General Bönnighausen and forced him to retreat across the Rhine. Landgrave William V died in the Autumn of 1637 and his widow, countess Amalie Elisabeth became regent for her eight-year-old son William VI. She held on to her late husband's anti-Habsburg policies. Melanders was no longer willing to support this stance and resigned the command of the Hessian troops in mid-July 1640. He was then courted by the Emperor.