Havana. It was in Havana that Parker recalls his ship almost running aground due to heavy winds off of Morro Castle. After spending the winter in Havana, Potomac set sail for the United States and arrived in Pensacola in February 1845. The United States was by this time preparing for the Mexican–American War, which, although yet undeclared, was already causing both the US and Mexico to mobilize troops. Parker states in his biography that he recalls seeing the 7th Infantry Regiment beginning to mass in Florida for the coming invasion of Mexico. In February 1845, Potomac sailed for Veracruz and for the rest of the summer practiced gunnery exercises off the coast of Mexico. In August 1845, the ship returned to Pensacola and then was sent north to Norfolk for major structural repairs. The ship arrived in Norfolk on December 20, 1845, and was put in lay-up with the bulk of the crew reassigned. During his midshipman years, Parker also contracted yellow fever on two separate occasions. The first time was in the summer of 1846, and the second infection was in 1847. Parker's private observations of yellow fever was caused by the mosquito. Mexican–American War In February 1846, Parker applied to rejoin the crew of USS Potomac and was assigned as a midshipman under the command of Captain John H. Aulick. Parker served as Aulick's aide, and remarks in his biography indicate that the Captain was very stern with the crew. Potomac sailed for Mexico in March 1846 and anchored off of Sacrificios Island (near Veracruz). The vessel was assigned to a larger blockade squadron under the command of Commodore David Conner. In May 1846, Potomac relocated to Brazos Santiago and put troops ashore at Point Isabel. It was here that Parker witnessed the Battle of Palo Alto with the assigned naval personnel manning batteries supporting the advancing troops of Winfield Scott. Following the engagement at Palo Alto, Potomac returned to Veracruz and anchored off of Green Island, where it supported the blockade of Veracruz. It was during the blockage that Parker participated in his first actual combat engagement, this occurring when Parker was attached to a detail refilling ship's water flasks at the Antigua River. Attacked by a small force of the Mexican Army, Parker's detail, as well as a contingent of United States Marines, was able to repel the Mexicans and withdraw with few casualties. The engagement