By 1821, Simpson was already a second lieutenant in the Chilean Navy, and in the same year during the Peruvian War of Independence he participated in the capture of the Spanish ships "Resolución", "San Fernando" and "Milagro" in Callao, being promoted to captain on 7 October 1821. Admiral Cochrane gave Simpson command of the , ordered him to Acapulco to harass Spanish shipping, and from there move on to California to purchase supplies.