Franklin Pierce, on horseback, in a Mexican War battle scene with Chapultapec castle and Popocatépetl volcano, in the background, engraved, c. 1852. Pierce served in the Mexican American War and was given a commission by President James Polk. He had the rank of brigadier general when he arrived in Mexico and commanded over two thousand men. Pierce had no military experience prior to the war and saw it as an opportunity to advance himself politically.