He had applied for an appointment in the dragoons, the designation of the one regiment of cavalry then a part of our army. His alternative selection was the Fourth Infantry. To this he was attached as a brevet second lieutenant, and after the expiration of the usual leave spent at home, he joined his regiment at Jefferson Barracks, St. Louis. Duties were not severe, and the officers entertained much company at the barracks and gave much time to society in the neighborhood. Grant had his saddle-horse, a gift of his father, and took his full share in the social life. A few miles away was the home of his classmate and chum during his last year at the academy, F. T. Dent. One of Dent's sisters was a young lady of seventeen, educated at a St. Louis boarding school. After she returned to her home in the late winter young Grant found the Dent homestead more attractive than ever.

This was the time of the agitation regarding the annexation of Texas, a policy to which young Grant was strongly hostile. About May 1 of the next year, 1844, some of the troops at the barracks were ordered to New Orleans. Grant, thinking his own regiment might go soon, got a twenty-days leave to visit his home. He had hardly arrived when by a letter from a fellow officer he learned that the Fourth had started to follow the Third, and that his belongings had been forwarded. It was then that he became conscious of the real nature of his feeling for Julia Dent. His leave required him to report to Jefferson Barracks, and although he knew his regiment had gone, he construed the orders literally and returned there, staying only long enough to declare his love and learn that it was reciprocated. The secret was not made known to the parents of the young lady until the next year, when he returned on a furlough to see her. For three years longer they were separated, while he was winning honor and promotion. After peace was declared, and the regiment had returned to the States, they were married. She shared all his vicissitudes of fortune until his death. Their life together was one in which wifely faith and duty failed not, nor did he fail to honor and esteem her above all women. Whatever his weaknesses, infidelity in domestic affection was not one of them. In all relations of a personal character he reciprocated trust with the whole tenacity of his nature.

In Louisiana the regiment encamped on high ground near the Sabine River, not far from the old town of Natchitoches. The camp was named Camp Salubrity. In Grant's case, certainly, the name was justified. There he got rid of the cough that had fastened upon him at West Point and had caused fears that he would early fall a victim to consumption. In Louisiana he was restored to perfect, lusty health, fit for any exertion or privation. He was regarded as a modest and amiable lieutenant of no great promise. The regiment was moved to Corpus Christi, a trading and smuggling port. There the army of occupation (of Texas) was slowly collected, consisting of about three thousand men, commanded by General Zachary Taylor. Mexico still claimed this part of Texas, and it was expected that our forces would be attacked. But they were not, and, as the real purpose was to provoke attack, the army was moved to a point opposite Matamoras on the Rio Grande, where a new camp was established and fortified. Previous to leaving Corpus Christi, Grant had been promoted, September 30, 1845, from brevet second lieutenant to full second lieutenant. The advance was made in March, 1846. On the 8th of May the battle of Palo Alto was fought, on the hither side of the Rio Grande, in which Grant had an active part, acquitting himself with credit. On the next day was the battle of Resaca de la Palma, in which he was acting adjutant in place of the officer killed. One consequence of these victories was the evacuation of Matamoras. War with Mexico having been declared, General Taylor's army became an army of invasion.

Volunteers for the war now began coming from the States. In August the movement on Monterey began, and on the 19th of September, Taylor's army was encamped before the city. The battle of Monterey was begun on the 21st, and the desperately defended city was surrendered and evacuated on the 24th. Grant, although then doing quartermaster's duty, having his station with the baggage train, went to the front on the first day, and was a participant in the assault, incurring all its perils, and volunteering for the extremely hazardous duty of a messenger between different parts of the force.

When General Scott arrived at the mouth of the Rio Grande, Grant's regiment was detached from Taylor's army and joined Scott's. He was present and participated in the siege of Vera Cruz, the battle of Cerro Gordo, the assault on Churubusco, the storming of Chapultepec, for which he volunteered with a part of his company, and the battle of Molino del Rey. Colonel Garland, commander of the brigade, in his report of the storming of Chapultepec, said: "Lieutenant Grant, 4th Infantry, acquitted himself most nobly upon several occasions under my own observation." After the battle of Molino del Rey he was appointed on the field a first lieutenant for his gallantry. For his conduct at Chapultepec he was later brevetted a captain, to date from that battle, September 13, 1847. He entered the city of Mexico a first lieutenant, after having been, as he says, in all the engagements of the war possible for any one man, in a regiment that lost more officers during the war than it ever had present in a single engagement.

Perhaps his most notable exploit was during the assault on the gate of San Cosme, under command of General Worth. While reconnoitring for position, Grant observed a church not far away, having a belfry. With another officer and a howitzer, and men to work it, he reached the church, and, by dismounting the gun, carried it to the belfry, where it was mounted again but a few hundred yards from San Cosme, and did excellent service. General Worth sent Lieutenant Pemberton (the same who in the civil war defended Vicksburg) to bring Grant to him. The general complimented Lieutenant Grant on the execution his gun was doing, and ordered a captain of voltigeurs to report to him with another gun. "I could not tell the general," says Grant, "that there was not room enough in the steeple for another gun, because he probably would have looked upon such a statement as a contradiction from a second lieutenant. I took the captain with me, but did not use his gun."

The American army entered the city of Mexico, September 14, 1847, and this was his station until June, 1848, when the American army was withdrawn from Mexico, peace being established. There was no more fighting. Grant was occupied with his duties as quartermaster, and in making excursions about the country, in which and its people he conceived a warm interest that never changed. Upon returning to his own country he left his regiment on a furlough of four months. His first business was to go to St. Louis and execute his promise to marry Miss Dent. The remainder of this honeymoon vacation was spent with his family and friends in Ohio.