He served as a colonel in the United States Army during the Mexican–American War, was brevetted to major general, and became known as the "hero of Contreras".

After his political ally, Edward D. White Sr. was elected governor, Smith was appointed state adjutant general.

[1] At the onset of the Mexican–American War, he was made brigadier general of Louisiana volunteers on May 15, 1846, and accepted a commission as colonel in the United States Army on May 27, 1846.

He was brevetted brigadier general on September 23, 1846, and joined Winfield Scott's army as commander of the newly created Regiment of Mounted Riflemen.

[4] After the war, Smith commanded the Pacific Division from 1848 to 1849 including as the 6th military governor of California from February to April 1849.

[5] He sent relief parties across the Sierra Nevada in the fall of 1849 to meet the last arrivals in the emigration, saving many lives.

[1] He served during the Bleeding Kansas conflict and organized multiple sortees of soldiers from Fort Leavenworth to reduce the violence between pro-slavery and abolitionist groups.