"He was an ardent admirer of John C. Calhoun, and eventually became his successor as the leader of the South. In his rare speeches in the House of Representatives he clearly defined his position in regard to states rights, which he consistently held ever afterwards. During his first session, war with Mexico was declared, and he resigned his seat in June 1846 to take command of the first regiment raised in his state -- the Mississippi Rifles. He served in the Northern Campaign under his father-in-law, General Taylor, and was greatly distinguished for gallantry and soldierly conduct at Monterey and particularly at Buena Vista, where he was severely wounded early in the engagement, but continued in command of his regiment until victory crowned the American arms. While still in the field he was appointed (May 1847) by President Polk to be brigadier-general of volunteers; but this appointment Davis declined, on the ground, as he afterwards said, "that volunteers are militia and the Constitution reserves to the state the appointment of all militia officers." Afterwards, Davis himself, as president of the Confederate States, was to appoint many volunteer officers."

http://www.nndb.com/people/336/000050186/