During the Mexican–American War, Leonard Ross volunteered in the
4th Illinois Volunteer Infantry Regiment, enlisting in June 1846. He was promoted from the rank of private to first lieutenant on September 4, 1846, by the commanding officer, Captain Lewis Winans Ross, his older brother. In a letter to his wife, Captain Ross remarked that Leonard "enjoys his honors very well & will make a very good officer. He lacks dignaty [sic] which I am imparting to him as fast as possible from the surplus which I had of the article on hand." Leonard Ross subsequently played prominent roles in a number of battles, including the
Siege of Veracruz and the
Battle of Cerro Gordo. Lieutenant Ross had sent in his resignation by that date and had already returned home from the war. However, it was also noted by Captain Ross that Lieutenant Ross' resignation was not accepted, so he was officially considered absent without leave, although there were apparently no adverse consequences. An extensive account of Leonard Ross' service during the Mexican-American War has been provided by the State Historical Society of Iowa. He was elected Colonel and fought in this capacity at the battles of
Fredericktown, and
Fort Donelson. and for a while led the 1st Division of the Army of the Tennessee. After a variety of commands on garrison duty, General Ross was placed in command of the 13th Division of the
XIII Corps. Based on Ross' service record, Grant then relieved him from his arrest and returned him to his command. Ross was placed in charge of the
Yazoo Pass expedition against the Confederate fortress of
Fort Pemberton guarding Vicksburg from the north. Ross' expedition amounted to little more than an artillery duel against the fort before he decided to abandon the expedition. At roughly the same time, reinforcements under General Quinby arrived, and Quinby convinced Ross to renew the attack. Ross did so with few results and eventually withdrew. Ross resigned his commission on July 22, 1863, in order to look after his personal interests, believing that the war was nearly over. A detailed description of Ross' service during the Civil War was published by the Iowa State Historical Society, ==Political service==