 McClellan 's first assignment was with a company of engineers formed at West Point , but he quickly received orders to sail for the Mexican – American War . He arrived near the mouth of the Rio Grande in October 1846 , well prepared for action with a double @-@ barreled shotgun , two pistols , a saber , a dress sword , and a Bowie knife . He complained that he had arrived too late to take any part in the American victory at Monterrey in September . During a temporary armistice in which the forces of Gen. Zachary Taylor awaited action , McClellan was stricken with dysentery and malaria , which kept him in the hospital for nearly a month . The malaria would recur in later years — he called it his " Mexican disease . " He served bravely as an engineering officer during the war , was frequently subject to enemy fire , and was appointed a brevet first lieutenant for Contreras and Churubusco and to captain for Chapultepec , He performed reconnaissance missions for Maj. Gen. Winfield Scott , a close friend of McClellan 's father . 
