After graduation from the Academy, Prince Pierre received an honorary appointment as an acting ensign in the United States Navy on 28 May 1863 and served on the corvette , a training ship for midshipmen in Newport; he joined its crew while it was part of the South Atlantic Blockading Squadron, stationed off Morris Island inside Charleston Bar. Unlike his father and cousins, who left the country on 7 July 1862, he remained in the service of the United States. He was assigned as a lieutenant on the "John Adams", but was not promoted to that rank. He regretfully resigned his commission on 30 May 1864 when Franco-American relations cooled with the French intervention in Mexico on the side of the Second Mexican Empire. He returned to France the next month.